For a several months now, my party of level 10 PCs have been on a collision course with one of the main factions of the large island the campaign is set on, and tonight things finally came to a head with a clash between their slave rebellion an contingent of the evil conquistador oppressors they are struggling against. Everyone was excited to see the fruits of all of the resource gathering, recruitment, politicking, and sabotage the campaign has focused on since beginning this arch, and the strike a decisive blow for freedom. Tensions were high as their forces of nearly 10000 untrained, poorly armed rebels prepared to give battle against a contingent of just 2000 professional soldiers, and there was serious discussion among the players about whether losses would be too high. But no, for the good of these people and their families who were seeking sanctuary in the nearby Fey jungle, these forces MUST be defeated. Alright, your soldiers are ready to fight and die for their freedom let's do this....and we did.
I first got an inkling of of how "off" this system was when I saw the completely 1 sided nature of the Battle Rating (BR) system. Their army of escaped slaves were each effectively Tribal Warriors, CR 1/8th, while the enemy's army was composed of professional soldiers, CR 1 with some CR 3 veterans serving as sergeants. With the limit of 400 individuals/unit, that meant that each unit of slaves had just 20 BR, while the soldiers had 250 BR. For anyone who hasn't read the rules, Battle Rating serves as a unit's main stat, and is used for both offense and defense. When an attack is made, both the attacker and defender make opposed BR rolls. If the attacker wins 1-10, the defender loses 2 BR. If they win by 11 or more the defender loses 5BR AND makes a Moral check to avoid fleeing the battle (more on Moral in a moment). As should be pretty obvious, the maximum roll of a d20+20 is 40. The lowest roll of a d20+250 is 251. There was no way that the slaves could EVER harm the professional soldiers, regardless of numbers. Maybe that is intended to make for cool 300 style stands against superior numbers, but when the PCs are the inferior forces, it feels like throwing people they care about to the slaughter. But ok, the PCs did a pretty good job of convincing some powerful groups to join them. This can just be a fight between the PC's strike squads and the enemy army. A little creative behind the scenes number crunching later (the PCs don't know the CR of their allies) and they are slight underdogs, but only by a bit. Or so I thought.
The second issue that quickly became obvious is how one sided the Moral system is. Moral is determined by a number of factors, but is basically "how likely is your force to stand its ground" and is modified by the Charisma of the commander of each unit. Whenever a unit loses a fight by 11 or more, it needs to make a Moral check with DC 10 or be routed. For professional fighting forces with remotely charismatic commanders, Moral should always be 5+. For a bunch of escaped slaves, the PCs had to pull out all the stops to just avoid negatives. At first this seems ok, so what if a few units break? As long as the groups that the PCs are part of (with their super human charismas) don't fail then the battle is still fine. The problem comes with the rule calling for Moral checks from EVERY allied unit within 500ft when a unit flees or else lose its next action. We've already established that the escaped slaves will always take heavy losses from a single attack, and with 0 Moral, they have a 50% chance to break. If they do, every other unit remotely nearby has a 50% chance to lose its action. In a fight were there might be 10 enemy units, each slave group in the area might have only a 1/32 chance to actually get an action, not just cower. More importantly, the important groups that the PCs have joined need to make these checks too. Even with a solid Charisma, they might have a +7 Moral. With 5 checks a round, there is better than a 40% chance that their pet unit isn't going to get a chance to do ANYTHING the next round.
You notice how I mentioned that the scale for Morale checks is 500ft? Well that is the next issue with the system: movement and distances scale by 10, but ranged weapons remain the same. In standard combat, it takes 10 rounds for someone to close to melee range with someone using a longbow. This makes killing fields horribly deadly and a serious deterrent to a frontal assault. In the Mass Combat UA, a round is assumed to take a full minute, which means the archers only get a single volley off before being forced into melee. For other ranged weapons, enemies can actually dash across the entire killing field without ever taking a shot.
But quite frankly all of those issues seem like little gripes, and could be worked with. The single BIGGEST issue with the system is simple: all of the rolling, tracking of BR and Moral, and movement around the map completely removes the focus from the PCs, which is what the game is meant to be about. A single PC's action might take 40-60 seconds of session time, while a single turn for a group of units could easily take 4-5 times as long between movement, making contested checks, moral checks, and then moral checks for everyone else in the area. Where the PC might roll 3-4 dice on a turn, plus a few from the DM for saving throws, the army units EASILY roll 20 dice in a turn, often with many different modifiers. Add in keeping track of which units can and can't act, and the book keeping becomes incredibly tedious.
And at the end of the day, there is absolutely zero payoff for all of this work. In a strategy game, careful deployment of troops, good use of tactics, and clever ploys can turn the tides of battle. With the anemic BR system, positioning only matters in cases where there BRs are within 19 to start with, which is extremely uncommon except in specific cases. In all other scenarios, there is no room for strategy, only for ramming your head against a wall, doing pointless book keeping, and stealing the spotlight from the characters the PCs actually care about.
Best Answer
Establish PC's goal in the battle
Never forget that the battle has a purpose but your PC's motivation might be different. Are they fighting to help win the battle to save a city? Are they forced to fight and want to survive together? Are they protecting an NPC friend who will fight no matter what? Do they defend their honor after swearing that the right flank will hold? The battle itself cannot be the goal, there has to be a well-defined challenge for the party.
Assure that players have competence to attain that goal
If they are just fighting to win the battle, they need something to differentiate them from a sea of regular troops. They can be commanders or officers, they can have their own cohort to lead or they could just be veterans instilling hope and courage in others. Otherwise they will have no agency.
Conversely if they wish to attain a different goal, such as defending their friend prince from death, they need to be put near him - or have at least a way to reach him.
Treat the battle like a scene that surrounds the players
It's easy to forget that the battle is just another way of shining light on the characters. Your players are assumed to fight near constantly, unless they can manage to disengage the opponents. Do not treat that as combat encounter, but rather as a puzzle that can be solved through leadership, cunning and violence. Instead of having your barbarian use his rage to fuel his damage output, let him burn it as a mean to attain advantage. So, using that as an example if your players protect an NPC - Alice, The Brave Maiden from death in battle, Brong the Barbarian might spend some of his rage to clear an opening to reach her in time.
Make battle events relevant to the players in their own scope
Which means that as the battle is going they seem to hold their own, but when the enemy sends reinforcements, they might have to fight an elite enemy as if it was a standard one-on-one encounter. If they rally their troops to exploit an opening, describe how lowly soldiers ravage enemy lines with their newfound courage. Also, limit their knowledge - they shouldn't have any idea what happens on the other flank, unless party rogue climbs the remains of a catapult to scout the situation.
Ensure party goals can be met regardless of the outcome of the battle
That's pretty straightforward. Give players an opportunity to carry mortally wounded Alice into the forest. If their flank collapses, let them be warned to cast invisibility to escape. If the fortress is being overwhelmed, let them carry the MacGuffin to safety in the nick of time.
Last but not least
Assume they act as a team like they would normally do
That means protecting each other, keeping formation, using their basic fighting skills to attack and defend. Don't force the players to tediously adhere to the battle grid or roll dice all the time. Just assume they are fighting a battle and throw in a juicy description how Cerion the Fighter shield-bashes a particularly large orc and Daria the Mage blasts him into a group of goblins. If you want to portray battlefield attrition, you can abstract their actions to a single "performance" roll, which would penalize them with wounds or depleting battle supplies for a bad roll, e.g. Daria would use one of her spells or Cerion would lose some hitpoints. You may even let them haggle with you to get out of trouble, offering to use up a spell or lose a piece of equipment instead of being wounded etc.
Mass battles usually abstract mobs as if they were single units. You have to do the opposite - abstract all the fighting chaos into landscape events not different from weather or sunset and have particular bite-sized challenges appear from it.
Refer to fictional battles as reference material
And the classic example of that is Battle of Helm's Deep - let's stick to the film version.
The characters have a goal - to protect people of Rohan in the fortress (and to win a kill count contest). They have competence and a role - they are leaders and champions, even though the battle is commanded by Theoden and elvish force by Haldir, they have an influence over troops around them. They fight all the time, but only interesting bits are shown on screen. They only are aware of their immediate surroundings and what happens at the gates is not shown while they are in the keep. They in the end achieve their goal and even if Eomer wouldn't come, they would still have hope for women to escape from the caves. Finally, they meet specific challenges - to hold the wall first, then to kill the famous Torchbearer Orc which fails, then to hold an opening for the elvish troops to regroup. When a battle takes turn for worse, Gimli and Aragorn heroically defend the bridge allowing soldiers manning the gatehouse to rebuild, and afterwards protect the king while he retreats to the keep. Then they convince him to lead one last sortie, while arranging for the women to flee through the mountain pass. Plenty of small-scale challenges in the midst of total chaos and what a battle to behold!