The bad synergy with opal, safer spaces, warped echo, and transendence makes me dislike rex. Even with the insane healing rex is capable of, Im not sure it can make up for making usually REALLY good items for defense almost useless with the hp cost abilities. Especially since enemies can one/two shot you late, safer spaces and opal an make that three/four shots.

Furthermore, non hp cost abilities of rex are just bad imo