If you hit with 121, why would you ever not cancel into Flamethrower, Low Shot, or Cobra/Wrist Cannon? It's such an easy confirm. 121 is only 7.5% damage, so trying on-hit stagger nonsense will almost never be worth it over cashing out.
Canceling into grenade will only hit an opponent completely unfamiliar with the mu, since it doesn't actually connect from 121. I believe a savvy opponent can flawless block the bomb, though I'd have to test. Either way, there's not much RoboCop can do with +1 from that range. I believe there's also a gap where the opponent can poke, which removes the bomb from play. I'd need to double check, though.
So, you're not wrong that 121 is strong and the hit advantage allows for some seemingly solid options, but I don't believe any of those follow-ups are worth the risk when he can just as easily cancel into guaranteed damage and still get most of those same setups.