It uses the audio tracks to do the sync. So if you have good quality audio, the sync will be very good, as long as the cameras are fairly close together to not have any audio delay.
I tested it on one of the multi-cam projects I did last year. Over 30 hours of footage, 3 main cameras, etc. Took me a couple of days to digest and sync up all the footage.
I tested PluralEyes on the first 1 hour of each camera and it worked. 2 of the cameras had a mixer feed and another camera was located on stage so still had a clear audio signal. However, one of the cameras has a noticeable delay because it was located much further so the sync wasn't perfect. This however is fairly easy to correct in the multiclip.
I'm going to test it out with other footage later on. At first glance, if you work on large projects, this app might be worth it. I'm actually surprised this isn't part of FCP already...