Ryu needs gowen spheres and a lvl3 support unit (=distruptable, field relieing) on the field to to do what it can. so sp wise his set up is equivalent to urgant (6 sp in lawtia), just has more punch, no sp drain and 10 hp more tankiness (which is needed probably on half of lvl4 falkow units anyway).
Ex mermaid is a very good tank no doubt, but i would not call it OP not when the game is full of cheap damaging attack skills, startskills, open skills and startskills.
Attack immunity is present in the game long time just neglected. It has its strenghs but no way is it Op, you just need to take a look to posses action skill carriing units when you design your deck. Manta long time trolled you if you didn' t do that.