A creature has to succeed on a saving throw to avoid the effect (PHB p. 223).

The target must succeed on a Wisdom saving throw or follow the command [...]

As your enemies grow in power, so do you. The saving throw is calculated as:

Spell save DC = 8 + your proficient + your modifier

The modifier depends on your class. If you are a sorcerer, it is the modifier as shown for your Charisma.

So if you are a level 17+ character, you are not unlikely to have a Saving DC of about:

8 + 6 + 4 = 18

If you have a way to increase your Charisma up to 30 (unlikely, though) then you would get:

8 + 6 + 10 = 24

In other words, the odds remain about the same when you are at lower or higher level. However, the Command spell will be strong against henchmen of a powerful foe since those guys are likely of a lesser level. Something to keep in mind... Especially if you use a higher level spell slot and command several monsters simultaneously.

Some powerful creatures have very high Wisdom and will succeed more often. But really, most monsters don't have such a great Wisdom to start with.

However, you are certainly more likely to encounter creatures that are not affected by enchantment spells or have some resistance to magic allowing them to better resist your spells in general.

1. If the Dash action is the fastest means of fleeing available to them, then yes, they will use it. For a counterexample, a monster with the ability to teleport (like a yugoloth) would probably do that instead.

2. If the Disengage action is the fastest means of fleeing available to them, then yes, they will use it. (I really can't think of a way to make this true, though.)

3. A creature under the effect of a Command to flee uses its own movement, so yes, it will provoke opportunity attacks.

   You also don’t provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction (PHB p.195).