Just a quickie… FCP X 10.1.3 works fine in Yosemite. Snappy even! No problems yet , and I’ve been hammering it for a while. Go for it!
Also, for what it’s worth… FCP 7, Adobe Pr and AE CC, Soundtrack Pro all good as well. No functionality differences from Mavericks as far as I can tell. Avid MC 8 appears to be OK, though I haven’t done much in it, and Resolve 11 seems OK too, though I had to switch the GPU to use OpenCL rather than CUDA. Might just be me though.
So… All good, dive in!. But, if it all blows up… don’t blame me!
Final Cut Pro X Babbling 
I upgraded to Yosemite and my Blackmagic ultra studio 4k spits out video out of Resolve fine, however my av output switch in fcpx is grayed out and I can’t click it on to get playback. Anyone get this? I am on a new mac pro and haven’t had problems thus far.
I’ve seen this before… Not sure it’s the same issue, but usually re-installing the BMD drivers has cleared it up.