>>2102326
There is the glorious achievements wincon too.
>>2101248
>>2101256
it isn't that the ai can't handle them, it's that peasants are the best cost/numbers ratio in the game barring regional/faction exclusive units which are at best even. That matters a ton due to the catchup faction revive mechanism, if you actually want to do a full 100% europe conquest you need like a full 900 peasants + assassin in every region or you have to really game the system by leaving enemies alive in one province and then taking them all down at once.
That said, I've done a lot of Blind King's mod which makes peasants require a building and the ai does still do simple armies, although it's not a big issue. The biggest difference is that I've seen is that the armies are smaller because they aren't spamming 100 size units, so it takes longer to hit 960, so the ai tends to have a lot more space to recruit as they get new buildings and more variety because of it. Although, in general the ai definitely has a problem overvaluing siege engines and guns.
>>2101237
They spam balistas because they're the first siege engine available. If you start in later eras you'll still see the same issue, just focused on demi-culverins or whatever other big gun they can get right away.