Well...considering that
doesn't really build a lot of damage besides for 1-2 items and then tanks the rest of the way I don't see it really being a problem. Yes his ult might hurt quite a bit if you were to build full AD Jarvan, but then he is super squishy and easy to burst down. If he jumps on your entire team and burst all 5 of them down, then that would be your own fault for sticking together in a small pile for his ultimate in the first place. Even if he builds full damage, chances are he had to use his Flag and Q to even get into position to even ult your team, and after that what is he going to do while waiting for his CDs? The 2 other champions you mentioned also doesn't really say much.
's ultimate is weak damage wise because of just how strong of a CC it really is. Being able to move the entire enemy team into a direction and stun them for a pretty big tank is huge. Sure Mini-gnar might not be super tanky, but to even use his ultimate he needs to be mega gnar.
's empowered Q is pretty quick to get and Idk why you even mentioned it since that's comparing Jarvan's Ultimate to Rengar's empowered Q. Comparing a 2 min CD ultimate that is basically the unique ability of Jarvan to a powered up Q which can probably be charged every 10 or so seconds.