Okay found it.
The previously mentioned critical bug in KB/HoMM minigame is
Training Grounds building.
Instead of
adding +10% on your recruits damage, it sets them to do only 10% of the full damage.
A curse instead of buff.
Probably instead of damage*1.1, someone erroneously wrote damage*0.1 in the formula.
This is outright horrible and whatever you do, just
don't build Training Grounds.
Other buildings seem to work properly.
There is an unofficial mod and it's yesterday's version fixes this critical bug and some other not really important stuff.
But it also adds features of the tabletop that might be deliberately removed because this is cRPG and I don't want those in my game:
https://github.com/Vek17/WrathMods-TabletopTweaks/releases
If you ask me how it is possible this wasn't noticed during beta, I have absolutely no idea.
This one is a huge setback (I don't know if destroying that building reverts damage back to default), not one of those small annoyances where okay this spell doesn't work as intended (some touchbased ones) so whatever I'm gonna use another spell.
Later or tomorrow I'll share some base economy and army building tips.
One thing is for sure, while I was studying this bug above, I've seen several persuasive articles/posts out there suggesting wasting tons of ress on recruiting Hellknights - maybe on the nightmare mode this is a necessity, but I'm playing on normal difficulty and I do not recommend this.
Always buy your normal, cheap, melee footmen and cavalry.
The minigame, at least in chapters 2 and 3, is all about archers and clerics, you can't have enough of those.