FCP X synchronization has an annoying limitation: it needs to use a clip as its base for syncing others, but it will only accept a video clip for this, never an audio clip (here, when I write "video clip", I mean a clip that has both video and audio). In other words, FCP X synchronization will connect video to video, audio to video, but never video to audio.
Let me explain with a real-life example:
I shot a one hour interview, using a DSLR and a lavalier mic connected to a digital audio recorder. I let the digital audio run for the whole interview, but because of file size limitations of my DSLR, I ended up with 5 video files.
I decided to use FCP X for this, as an experiment. So I import my media in a new Event. I select all of my clips and launch "Synchronize clips". When the sync process is over, I giddily open the sync compound clip to see that it failed: the first video clip is in the primary storyline and has the audio clip connected to it at the proper point in time, but the remaining video clips are connected to the first video clip in a seemingly random fashion...
What happened? Why did FCP X try to sync my video clips to each other but failed to properly sync them to the audio clip? I've run a few tests and come to the following conclusion: while syncing, FCP X will always assume that a video clip, and never audio, has to be put in the primary storyline and other clips will be connected to it.
The workaround is to repeat the synchronization operation for each video clip, which isn't very efficient. This shows that even though FCP X can do synchronization, it doesn't even come close to what a 3rd-party tool like Plural Eyes can achieve.