While FCP 10.4 does have built I support for 360 footage, it is necessary to use the camera manufacturer's stitching video to stitch the 360 video before trying to import it into FCP X. I have an Insta360 One 360 camera, and FCP X will not import the native camera footage without first taking it into Insta360 Studio and outputting it as a stitched h.264 file. Then I can take it into FCP X and edit the 360 footage. Adobe's Premiere Pro has a plugin for the Insta360 One that allows the native footage to be directly imported into Premiere Pro CC without the intermediate step of stitching it. Unfortunately, FCP X does not have such a plugin. Thus in between the footage captured in the camera and the output of your edited 360 video there are at least two compression cycles (and if you post on YouTube another compression in YouTube) which really lowers the quality of the final 360 video project. It is early days for 360 video, and I am hopeful that Apple and 360 camera makers will cooperate together to resolve these issues.
Tom