This is a known issue with FCPX and the 680mx card in the new Imac. Apple is working on a fix, but for now avoid titles, causes it the freeze and crash (unless it is rendered quickly).
Hopefully is gets fixed soon, because this is quite annoying.
Wow the bad news seem to never stop.
First the poor build quality, then the long delays now final cut.
I must say I'm on the same boat as you guys. I wanted to use it among other things with final cut. I just hope apple fixes the problem soon.
The good news is that since I'm still waiting for the delivery of my machine I won't have to suffer through Final cut's crashes! 😀😀