I use a thumb throttle, I really like it, and I get to keep my nice looking, comfy grips. I have no trouble holding on to the handle bar, don't let that scare you. I ride single track, downhill, rocks and rough trails all the time.
Lots Quads and ATVs use thumb throttles, no problem there either. Twist throttle can be very dangerous. How many videos have I seen where someone is learning to ride a motorcycle and they fall off the bike but still hold on to the handlebars causing the bike to rocket away.
Also, bumps and body movement dont cause as much variation in throttle input as your hand is fixed to a the solid non-rotating bar.