Motion...Compressor...Qmaster...Cinema Tools. The ones that ship with Final Cut Studio...they all stopped working on Yosemite. They still don't work under El Capitan. Final Cut Studio was discontinued 5 years ago...every OS update after that doesn't take it's functionality into consideration. If it breaks, they don't care. Apple has a new version of FCP and Motion and Compressor that it supports.
Now...FCP 7 will still work, but not as fully capable as it does under supported OS versions. What doesn't work? it's not consistent. People report sync issues, others can't export QT. Some report title tool (Boris 3D) issues.
If you need Final Cut STudio to work....then you need to keep an older OS (and typically older computer, as newer computers can't run older OS versions) in order to do so.