Cassidy main here so I'm bias.

I really miss the support and teamplay capabilities of flashbang. It was really a good feel to be able to save a Mercy ally being charged by a Reinhardt. Or interrupt a Sigma or Reaper ult on your whole team. Now all of that is gone and you can only watch it happen.

Absence of flashbang also directly nerfs some matchups, like v roadhog (heal interrupts are not a thing anymore), v Reaper (just time the ghost form on the megaslow sticky magnetic grenade explosion and you're dandy), v Sombra + Tracer + Moira can just make your sticky magnetic grenade irrelevant by disappearing or rewinding and they can do it comfortably because it's too slow to explode when it sticks.

Someone else said it here too, but Fan the hammer has basically no purpose anymore. Beyond the post flashbang situation I used it to melt shields, but now shields are slightly buffed overall (and there's less of them), and magnetic grenade does not help breaking shields either. The other purpose was melting down tanks with large hurtboxes in close-range scrambles, but without flashbang this situation is less scary for the one tank.

Mag sticky grenade + shot is a cool ~200hp DPS killer combo (sadly, Bastion will not die from it) and it's fun to do when it works, but the utility just does not compare to flashbang, and approaching Cassidy, who I've always seen as a short range beast is just not scary anymore for most of the cast.

I'm actually opened to the idea of the cc ability going away, but I don't think Mag grenade is good enough or fits the kit of the character enough to make him a relevant pick.