Magneto's ult is too conditional

He has the only tank ultimate that can be stopped outside of being CCd in startup, or being outright killed. Once Hulk, Peni, Thor, Strange, Venom, Cap, or Groot get past the start up and their ultimate actually goes off, you can't stop them from having their effect happen. Hulk is GOING to be big, Thor is GOING to come back down, Peni is GOING to be running around, Cap is GOING to get tons of health regen. Magneto? He MIGHT not get his ultimate overloaded and completely negated. He has to deal with the same parameters of not getting CCd before it goes off, yet it can still be stopped by doing a lot of the 1 thing he's trying to stop you from doing, shooting your gun. Punisher should be absolutely hard countered by his ultimate, yet it's a complete risk/reward mind game that you either toss a wet napkin by throwing it early, catch the right amount of bullets and MAYBE kill a backliner, or hold it slightly too long and just fall back to the ground with 0% ultimate and not even toss it. It's already extremely easy to shield or dodge/go behind cover by the time he's absorbed enough bullets (if you're even shooting) to kill anyone. If they're worried about Magneto one shot potential, cap the damage at max bullet absorption to 225 so it can't kill full hp heroes, but add a slow so you can finish them afterwards or kill them if they're hurt. That way you cannot get 1 shotted, but it's still scary enough to want to avoid it as well as not shoot your gun. Right now your options are: Don't shoot, shoot a lot, hide behind a shield, get cover. The only way to effectively die to Magneto's ult is to be extremely out of position AND shoot your gun.

TLDR: Magneto is the only tank that has an X factor with the overload mechanic to think about outside of getting stunned during startup or killed. Remove the overloaded mechanic and replace it with a cap on damage that brings squishies to 1 shot at max, while adding a slow from getting hit so it's effective but not cheese.