He isn't trash tier, but he's kinda close. He has a decently low clear speed, which definitely limits him. I have a friend who played Sion jungle for a few games in a row with me, and his ganks were pretty worthless, but his late game tankyness was extremely useful. I wouldn't use him over Amumu, or even Sejuani, but people calling him useless isn't really that smart.
I could main Quinn or Galio and dumpster enemies because they don't understand what the fuck is going on, but it doesn't make them good. It's pretty much the same with Sion jungle. Literally just picking a counterjungler into him (Nunu, Eve, Lee, etc.) can kick his ass early enough to make him less useful later.