what looks like is happening is: FCPX uploads your video direct* to youtube, then it waits while youtube processes your video. Considering FCPX creates extremely high quality h.264 output for youtube (everything, actually), it can take youtube a relatively extraordinary amount of time to process the upload.
If you click on Share Monitor, then turn down the disclosure triangle on your Project ("folder") and click on the circled 'i' to the right of the exporting video, you'll see at the bottom of the pop-out XXXX Video Sharing.mov to YouTube XX% complete (after FCPX hits the ~95% mark). ...Progress is s...l...o...w...
From inspecting the log files, it looks like youtube and fcpx are communicating while youtube finishes processing your video. Testing a vid, I went to my youtube video manager page and sure enough, my upload was listed with a (processing...) on it. There is no graphic representation available of the stage of processing like the normal upload page gives. [Even though YT was indicating that the upload was complete and it was processing - when I cancelled in FCPX (@95% / about 10percent processing on YT's side) - it killed the upload to YT.]
Whether or not an upload succeeds... well that's a whole other kettle of fish. I don't have the patience to wait it out. Besides, I know how to optimize videos for youtube, and FCPX direct is about...oh... 4 to 5 times overkill... in most cases.
*the first thing FCPX does is render the project out in H.264 format before uploading... It does not do that "on the fly".