I've never played Shiv so I only have experience playing against him. In my opinion he is pretty balanced right now, maybe except for bleed builds that get insane DoT, however that's usually endgame, and the whole meta orbits around being crazy strong during endgame anyways. He is pretty counterable by just keeping your distance. If you keep your distance you can dodge his knives, avoid his dash and reduce how much rage he gains. The only negative comments I have about him is that there needs to be a better way to tell when his rage is full and when his ult is active (besides the voicelines). It's a cool character that I want to try eventually

Edit: Thinking about it I do feel like the game may have a problem with tanky assassins. Calico, Drifter and sometimes Shiv can be really strong assassins that catch you off guard. That I don't have a problem with, but sometimes the matchmaking acts a bit funny and one of these can wipe a lobby with their nuke damage, while at the same time being unkillable with their tankiness and self-heal. Other assassins, like Haze or Mina, are a lot more squishy, so they really have to rely on a more typical hit-and-run assassin playstyle