Where it fails are tree roots, fist-sized rocks, and tight going. It doesn't take much of a tree root to stop it cold (while your body continues forward), and I had to turn back from a rocky fire road in Middlesex Fells because I couldn't steer with all the bouncing. I also found it hard to control on somewhat smooth but tight and twisty paths through trees (Belmont's Rock Meadow/Beaver Brook North), but that might be okay with flat bars or dirt drops rather than road drops (the problem was brake leverage and getting my hands bounced off the hoods).
It's an eccentric thing to do, no question about that. :-)