He gets full punished on block. Its a 50/50 for him too.
this! ... actually his 50/50 is quite easy to punish with a full combo. plenty of characters can dish out way more damage than Ermac so it is not so much in Ermacs favor as all these narrow minded people think. in fact Ermac's real strength is the amount of meter he usually has, because he doesn't need it for combos or to ex specials. this meter can be spent on a breaker if Ermac should guess wrong.
one could argue that this whole design of Ermac is stupid; and I might even agree to some extend. But is it "too strong"? - Of course not. In fact every sane person sees Ermac far from top tier.
Ermac has plenty of severe flaws, like he has a damn hard time getting in. besides he can hardly open up the opponent once he got in (apart from the 50/50, which also has a rather slow start-up and should only be used after a successful poke). his whole defense sucks... and I could go on and on.
actually in most cases forcing the opponent into a guessing game is some sort of "last resort", as most of the time this risk is what Ermac wants to avoid as long as possible. I could explain why in more detail, but then again all these narrow minded people won't think any further than "oh, a 50/50 and I could guess wrong thus it has to be broken" anyway.