I've fallen hard ~5 times over the course of ~1000 miles, but it's a lot less frequent now that I've learned what to watch out for. You do need to be
very vigilant on rough/potholey roads + light rail tracks, and very aware/defensive around other vehicles, of course.
If you're going fast in a straight line it can handle a fairly rough road. The slower you're going the more likely an obstacle is to stop the board and throw you off, but of course the faster you're going the more it will hurt if it does :)
Carving + powered wheels + debris also makes it more likely for the board to slide out from under you.
Basically avoid all potholes/rough patches/sand/etc at first and over time take them on more aggressively as you get comfortable. When in doubt go around or stop and walk across.
And wear a fucking helmet.