My swb recumbent fell out of the bus bike rack once.... 20"/26" wheels. I had the 'swing arm' over the front wheel, not over the boom. The bus driver was driving like a total jackass, turning at speed, hard braking, speed bumps at speed, etc, so that did not help. Luckily it jumped out on a speed bump going fairly slow so the only damage was a bent bolt on my chain pulley which was replaced. I now keep the arm on top of the boom and put some bungee chords around my rims and the rack to stop the bike from jumping again.

My 700x40c wheeled bike doesn't sit that well in the wheel wells on some of the older looking racks. It has trouble swallowing up the diameter. Seems like they are designed for 700x23c and 26x2.0 tires. I also have a Racktime Topit front rack which prevents the arm from resting on my wheel (too be fair, so does the fender). I wedge the arm in the nook created by the mounting tab of the front rack and fork. More gratuitous usage of bungee chords. No issues even on the interstate... yet.