It's decent, I'd say. Archers aren't that effective, so there's not much point in hiding them. Praetorians and monks perform much better.
If the enemy wants to kill the centaurs (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 play any role. I checked it in a duel.
2) Against neutrals, 25 HP is quite significant. + it increases the effectiveness of regeneration to some extent.
Not everything is decided in Fortress.

Барон "Vampires"
Отдай свои прошлые сны,
В сомненьях себя обрети,
В бешеном танце душа кружится!
Не думай, что будет потом,
Здесь и сейчас мы живем,
Стань хоть на миг сам собой, слышишь!