You are still talking about shit while I got owned in campaign mission 4. AI pulls heroes with lots of units out of their ass whole mission that I barely win over and then comes with unkillable hero with 100 crazy spells... this reminds me why I usually play skirmish in these games.
Another thing that's very true to HoMM (and similar games), then...
There's basically one correct way to do a mission and if you don't do that correct way, you'll get shafted - but often enough half an hour after you made the initial "mistake".
I'm not a fan of this, either.
I "fondly" remember that one mission in AoW3 where you could do very well and practically own the map, but at some predetermined turn, full stacks of AI would spawn at a certain location (right in the middle of your realm, far away from any threat so you won't protect them) and if you don't have armies to greet them, they'd waste your cities.