I've several Greevils, using my Seraphic with the fairly lucky combination of Hook + Purification + Phantom Strike. The game mode seems to be much better thought out than vanilla Diretide and has fewer game-breaking bugs, no lane farming, no massive imbalances, no mandatory boss killing in the end. The games are pretty fast too, although I played games where it was back and forth teamfight mayhem for the first 5 minutes without anyone getting a score.
I imagine having a bad Greevil would hurt a bit, but at least the randomness of items keeps the chances of winning a bit more even and you can always make up for it by picking huge teamfight heroes.
If you're new to the mode, remember to change between your hero and your Greevil often. They have separate health/mana, they regenerate while not being used and their cooldowns count down. Also, your Greevil is often a lot tankier than your hero so starting in that form might be a good idea. I randomed Storm Spirit earlier and could just zip away all my mana, blink around as a Greevil for a while and switch back to full mana SS.