Here's what you're trading for the alertness feat if you don't take the abil score upgrade and instead take the feat:
- +1 to damage
- +1 to hit
- +1 to AC
- +1 to Dex saves
- +1 to Dex checks
- +1 to init
The question then, ultimately, becomes, is +5 init (net +4), no surprise and no advantage on attacks against from hidden opponents worth it.
The latter two features are very dependent on your campaign and DM. Surprise should be a fairly frequent occurrence on both sides of combat, but that still depends on how willing your DM is to allow it or stage his monsters to have them obtain it (and kind of how cautious your group tends to be). Monsters attacking from hidden is another one that really depends on your DM, you group and how well you guys are at nosing out monsters from the shadows.
So ignoring those two items since they are DM and campaign dependent (and thus can't be weighted objectively), the question then becomes, is +1 to Dex more or less beneficial over 4 levels than +5 to init.
For pretty much any character other than the Assassin Rogue, I'd probably argue that they should take the stat upgrade early, the feat at L8 and then the last stat upgrade. However +5 init has amazing synergy with the Assassin Rogue and you're going to get a ton of mileage out of it.
If you're already at 20 Dex (if you managed to start at 20, or 18 and are at L8), then there really isn't any stat upgrade that is worth taking over this feat (though there may be better feats). Rogues are skill monkeys, yes, but +5% in a single ability set of skills is not worth the +25% to initiative and other features o this feat. The only one that would be worth considering at all is Con for the extra HP, but it's really not that many and you're better of boosting your damage significantly.
Take the feat at L4 (or even better, play a human and take it at L1).
I wanted to leave THIS here for reference and to let everyone reading know that NOTHING about the Revised Ranger Unearthed Arcana has been tuned or explained as it applies to Multi-Classing yet, as stated by Crawford. A single dip into this current ranger build allows considerably powerful combinations, such as permanent sneak attack.
That being said, and knowing that we have no SPECIFIC rulings yet on how to Multiclass this 'still in testing phase' Ranger, what follows are the rules for Multiclassing and how they apply to ALL Class Features. Of this there can be no argument until we hear details on how to Multiclass the Enclave Ranger.
Class Features only apply when levels in that Class are gained.
First; both of these Features are exactly that. Ranger Features; and as such the extra Hit Die and the ASI to your ranger companion do NOT work if you gain levels in other classes. Read this as 'For every Ranger level after 3rd'.
Example: Rogue Expertise
"...at 6th level you can choose two more of your proficiencies to gain this benefit..."
When you are a rogue you gain Expertise at 1st level. You don't then get to choose a second set of skills to gain expertise in at 6th level, unless you have 6 levels in Rogue. Even though it does not say as much explicitly, it is implied that this is a Rogue Feature, and applies only to Rogue levels.
The Feature you are asking about is Companion's Bond, and appears on Pages 5-6 of the Revised Ranger Document found in the Unearthed Arcana section of the D&D website. Since this is a class feature unique to the Ranger class Beastmaster Enclave, its rules only apply when you gain levels in Ranger.
Second; Ability Score Improvements (ABIs) are also class specific Features. So if you gain your ASI from the Ranger Ability Score Improvement Feature at the Ranger levels shown, your Ranger beast companion benefits from it as well. What this does not mean is that if you gain levels in, say, Fighter; which has more ASI granted to you; that it applies to your Companions Bond Ranger feature. Think of this as, if you are learning the art of the Fighter on the side, any experience you gain as a Fighter is in exchange for growing better as a Ranger and working better with your Animal Companion. Your Animal Companion is only ever linked to Ranger.
Same goes for any Features found in the Ranger list. Only Ranger levels. This can be backed up more solidly by looking at PHB pg. 163 - 164. The Multi-classing table has a definitive and exhaustive list of things that are gained when multiclassing, and points out that anything not listed there, or explicitly tied to Character Level elsewhere (such as Proficiency Bonus), is never tied to Character Level.
This makes a few solid points
- You gain Hit Dice and Hit Points only from your Class Level.
- Proficiency Bonus is based on Character Level.
- Only when you gain a level in a particular class, do you gain features from ONLY that particular class; both from any features given to you at that class level, and features that grow more powerful as you gain levels in that class.
The only exception to this rule, because 'specific beats general' is when it comes to your Animal Companion's proficiency bonus, as stated in the separate box on page 6. It says as you gain levels and your proficiency bonus goes up, so too does your animal companions proficiency bonus. It will always match your own proficiency bonus no matter what Multiclass levels you might have.
Best Answer
Ability score improvements are a class feature, and are only gained when the class level grants it. Different classes gain these advances at different levels (the Fighter is the clearest example of this), and there is no unified table of advancement by character level.
It's possible to never gain an advancement by always multiclassing into a new class before reaching 4th level in a class. One effect of this is that "dipping" a class for a single level or two is less obviously useful; considering the extra limits on what is acquired from a new class's first level when multiclassing, this is likely a significant reason for the design, apart from being able to give classes different advancement rates.