I've only ever used my Xero mounted on the rifle and I think it's missed maybe ~3 shots over the something like ~400? rounds I've shot over it. Being able to mount it to the rifle is what pushed me over the edge to buy one honestly.
What's better than data collected by the latest cool chrono? Data collected with the chrono placed in the same spot and chrono-to-muzzle relationship for every single shot, that's what. Plus, I can run it when practicing on props/obstacles (which is what most of my practice consists of these days) and it gives me the ability to throw it on for a stage if I'm shooting a match and something about my dope seems wonky.
It does display the error message from too much movement when getting into positions many times (and that may explain my literally few errant shots that didn't get picked up), but 99.9% of the time, if I'm actually doing what I should be doing and getting the rifle stable before I break the shot, it (nearly) always gets it.