I'm shooting a music video with 9 angles. Two of them need masking effects to add another clip in a doorway to the "outside". So far the only luck I've had to get the mask is to edit the multi cam to all the angles I want, then convert it to a compound clip. Next, line up a copy of the masked clip on top of the original by opaquing the new file so I can line it up. Anyone else had luck with a quicker method? FCPX won't allow mask effects on a multicam file.
Stabilization, tracking, optical flow smoothing, and auditions do not work in multicam clips. You have to open the clip and blade the timeline at the begin/end points of the multicam, apply the effect there, then hop back to the multicam and observe the effect.
At least the current version of FCPX now preserves the playhead position when jumping from the MC clip into the parent clip. Before 10.3 you had to write down or remember the timecode then enter it manually to find the corresponding playhead position between the MC clip and parent clip.