I've been using this thing for bikepacking on everything from flat dirt roads to techy singletrack descents and it just feels solid! No bounce at all! It will start to rotate around bars very slowly on sustained rough stuff but so far I haven't had to adjust it more than once a day. Plus you can remove the dry bag from the mounting system with incredible ease so you can pack it when it's not attached to your bike, which is much less awkward.
This thing is a gem! It rides really nice with the rigid arms that hold the bag above your front tire. Many other brands are floppy. Doesn't budge on bumps. Also, really like the bungee for attaching a speaker while riding on the road.