The difficulty of stabilizing a compound or multicam clip or retiming it with optical flow is a long-standing deficiency of FCP X. Considering how well FCP X otherwise does multicam, it is a glaring issue.
A workaround (developed by others) which works for both multicam and compound clips:
1. On your timeline, select clip then Shift+F to locate the clip in the event browser – this will highlight your clip
2. Cmd+1 – go to event browser window, use skimmer to check starting/ending time codes of highlighted clip & write them down. If needed first do View>Show Skimmer Info to display timecode above skimmer. Zoom browser in/out if needed to see clearly the start/end points of the highlighted clip.
3. Double click the multicam clip in the event browser to open it in Angle Editor.
4. Once in Angle Editor, press Control+P to type in the remembered time codes. Don’t enter colons, just the numbers, e.g: for 12:52:19 just enter 125219, then Enter.
5. Blade that section of the clip, then apply stabilization or retime with optical flow.
6. Press the “go back” timeline history arrow (upper left of timeline) to return to multicam/composite clip
Complex as the above procedure seems, once you’ve done it numerous times it becomes pretty fast. However it is a sad situation to be forced through such gymnastics on an editor which emphasizes ease of use.