Predictability of the AI.

I prefer one good hero over two not-so-good ones.

Why not-so-good?
I read somewhere that two heroes are better than one... I don't remember where... Well, the point is that as you level up, the amount of experience needed for the next level increases. When you have a hero at level 21, they need a lot of experience, and it's harder to level them up. But two heroes can develop well up to level 15 without any problems, while one hero will only be at level 18. :)

Added 2 minutes later

Guys, how many heroes do you take? I always take two... one tactician, the other a mage...

That's right. When I play as Life, I take a tactician and a mage. But then I decided that against some factions where you can get a main warrior hero (Nature, Barbarian), I'll take a Nature warrior instead of a tactician (it doesn't spoil the morale!). Or, you can sacrifice magic (against Barbarians). Level up a tactician and a warrior. In that case, the army will be very strong, especially against the Barbarian. But only against them. :)