Well, it's an ultimate, so it's not always on. And it's far from the cheapest ult there is
Soldier76 is a good one to learn on, but also he's a powerful character in his own right - he's got good damage, decent range, unlimited sprint so he's more mobile than many characters, can heal himself and everyone nearby acceptably, and really, his ultimate is decent but not gamebreaking. I mean, it's not like he sets off an explosion that will kill everyone in line of sight up to 20 yards away like D.Va, or becomes an unstoppable juggernaut of pain like Bastion or Winston. Basically, he's a good choice for most any situation, even though there's usually a character that is more suited to each situation individually. Snipers are better at range, tanks have more HP, the support characters heal better as a general rule, Tracer is faster, Bastion digs in and defends better... but in a game where the situation is fluid and changing rapidly, often enough Soldier76 is good enough to get what you need done, done, and push the flow of the game back in the direction you want it to go.
At least, that was my brief experience.