Regarding to the other rectangle units of the game, I found the Flesh Ripper weaker. Most rectangle units in the game have more than 1 health AND a special ability power, while he has only 1 health AND his power just allow him to gain the health that the other rectangle units already have in other factions...

Ok, he is a Initiative 1 unit (it makes him more efficient than other rectangle units, and he should be able to deal some damages before the other rectangle units...) but it seems stronger to me to deal damages two rounds later with a great special ability and 3 health !

Does someone has an opinion about that ?

The races which have rectangle units with more than one health either don't have a hex unit or their hex unit has less than 4 health. It evens out. I find that having a fast rectangle unit with the possibility of 3 health in addition to a hex unit with 4 health is fantastic. I have only played two games, but when I played as the Uthuk Y'llan, the Flesh Ripper was my favorite, most versatile unit.

Also keep in mind it takes only one Orb result to give all Flesh Rippers three Health

So they're best used in a bunch of 3+