There's been many many roleplays done from discord. Ok, not that many, but I've done a few dozen myself, though I managed to go down the 18+ road on most of them.

The main problem to me is that it is indeed perfect for fast paced roleplaying, and the only thing you see flawed is what gets me the worst. Too live-action, and it's easy to get in a culture of expecting frequent posts. That site damn near murdered my schedule that way :p