I think that soak tests are generally final software, but it's fair to say that there rarely have been issues. The biggest that I know of, last year's 4.4 update for the 2013 Droids, ended up "bricking" some testers' phones (but not everyone's.) Motorola had to help them get their phones back, some with warranty replacements. The soak test a few weeks ago for the VoLTE upgrade I understand didn't cause issues, but was reissued because the first one wasn't working properly.