Heroes 6 - Discussion

This is actually a very significant bonus.

It's a nice bonus, I'd say. Archers really don't offer that much, so there's no need to protect them. Praetorians and monks perform much better.

I saw a heroine among the Water faction who gives +25 HP to cannons. That's also very powerful.

No. If the enemy wants to kill the cannons (and they will, otherwise one of their strongest units will take three or four hits at a time – this is guaranteed destruction, even the AI understands this), then 25 HP won't make any difference. I tested it in a duel.