I am restoring Mavericks right now. While I didn't experience any lockups in the last two days I was starring at a black screen upon restart after switching the FireWire port connections of two of my monitors while the computer was off. None of my 6 monitors displayed an image even though I repeatedly restarted, unplugged, zapped the PRAM, tried to launch internet recovery mode, tried to launch the safe mode etc.
The problem is that there is no way to sign in or do anything else if you don't have a picture and the only thing that brought it eventually back was disconnecting all but one monitor, zapping the PRAM and launching recovery mode numerous times until it re-appeared. After re-installing macOS 10.12.4 the MacPro would crash after every login attempt and restart.
Furthermore connecting additional Displays caused the black screen to return resulting in further forced restart until I ended up in recovery mode again with only one monitor attached.
This black screen issue was reported on by other users when 10.12 was initially released. It's still an issue in 10.12.4.