Final Cut X is working fine here on OS X Yosemite 10.10 on a 2010 MacBook Pro 13".
Haven't tested 3rd-party plugins yet but so far everything runs the same as it did on 10.9.5.
Hopefully FCPX Libraries are safe on iCloud Drive, since they get corrupted on Dropbox:
"Sometimes you might not be able to render or share a Final Cut Pro X project. This can happen if you transfer a library via a third-party file sharing service (such as Dropbox)."