The weakness of all the ultimates listed above is that they slow the heroes down to a crawl, with the biggest offender being rocket barrage which literally makes Pharah sit completely still. Coupled with the fact that the ultimate makes them a huge target they can rarely, reliably get more than 2 kills at a time with them. Reaper can easily get stunned by a McRee flashbang and McRee can easily get fucked over by a Reinhardt shield, both non-ultimate abilities that are easy to use to counter the enemy. Genji's ultimate on the other hand actually massively increases his damage output and speed, coupled with the fact that he has a VERY wonky hitbox it becomes nigh on impossible to easily shut down a Genji ultimate, his biggest counter really being Transcendence, a support ultimate which is considered to be extremely valuable, and only has a limited range, so assuming you're not all grouped up Genji can still pick off a few heroes in the back lines or spread out as you may well be in a map like Hanamura.
I feel like the fact that Genji is pretty much a required pick in most high level and competitive matches really speaks volumes about the strength of the hero. While we were at insomnia we got to watch a few matches of the Overwatch tournament, and well, the hero is just plain broken from the slightly above average skill levels up to the highest levels of play. You know what we never saw? Pharah, even with her 'no skill broken op' rocket barrage ultimate somehow Genji still managed to be the preferred DPS hero for EVERY. SINGLE. TEAM.
I feel like complaining about rocket barrage, deadeye or death blossom is very much akin to complaining about Bastion.