Long time fcp user (since V2 I believe) but mainly an audio guy logic / pro tools etc.
I have a repeatable problem with audio in FCPX. The first time I start playback on the timeline after changing an effect or title setting, I will invariably get an audio glitch, like a half second buzz, around 3-4 seconds into playback. Rewind and play again and it's all fine.
I'm also getting a few instances using the titling where FCPX forgets the font size! I add title, change size, preview fine, then it auto renders with a font size of 0!