This is just not true. In the play-ins Sigmas were constantly getting kills with flux, either directly, or by just firing at the sitting ducks caught up in it. A hero suspended in the air about to land and be stunned in a predictable location with at best half health, is just dead no question.
At least with grav it can be eaten, or because everyone is together you might have a shield. Or with Shatter, it can be blocked. Nothing stops Flux, not even killing Sigma, and unless your team has beat or tranq, you’re probably dead.
The game needed fewer overpowered ults deciding fights, not more. Sigma’s entire design is one mistake after another and he’s OP. There’s a reason he was on both teams in almost every game despite being the hero they’ve all played the least.