I think that this issue is also in MoM, but maybe to a lesser extent. It mostly happens when you play on Impossible difficulty since the AI cheats and makes tons of units.
I usually just make border town with a large army to absorb all the attacks. Having good combat spells such as flamestrike, phantom beast, etc allow you end the battles pretty quickly.
Even if this problem isn't totally fixed by the game launch, I am sure by tweaking the number of opponents, difficulty, and loading some mods you will be able to find a balance that minimizes tedious micromanagement.
Well the AI in MoM as I remember it, were alot better at creating better units, especially like archers etc. Also because the span from rubbish units to great units were... less visible.
With games like MoM you gotta pay close attention to the spells you create, since they EASILY go and take control of them game, I have been gruing for the spells of mass destruction in the vids, because it seems at some point in the game everything is going to be controlled by the amount of mana each side has, and seemingly as I can see on the mana costs you are supposed to have thousands of backup mana easily available.
I would hope at some point that mana will be less fluctuating between having a MASS of mana, and struggling with having enough mana.
Same with spells, I am not in favor of strengthening the current benefits of having mages as heroes, as long as you have mana available, since it favors the player which have been the most lucky with the random placement of mana nodes.
Npc swarm ATM is not a problem to me, except I miss the AI players doing something properly with they're stacks of heroes and peons, Usually seeing them wander aimlessly around, a few time getting to the point of attacking, where rushing 1-2 defender units to my otherwise empty skeleton-crew cities is enough to kill 2-3 stacks of pointless peons thrown at them (And settler units for some reason, which kind of insane general sends settlers to invade enemy cities?
).
But I think it is as it is because Frog need to polish the AI, so thumbs up for posting, and ATM I only want a "Workshop" So I can design some lategame units for the AI, instead of having to play each faction in major games and going for specific unit strategies.
Also am excited to the next patch as there should be more (Less pointless) Traits to units in the design screen, either that or they just polished them 
Sincerely
~ Kongdej