Recently, I've been asked a lot from some editors with the problems of directly uploading files to YouTube from Final Cut Pro X. In response to them, I did some research and found that the problems were resulted from Google.
In early April 2013, Google has newly adjusted its APIs which control uploading files to YouTube. The changes set barriers to all automated upload systems, including Final Cut Pro, and thus one cannot upload files to YouTube from anywhere except Web Browsers. However, later on April 19, Google again changed the APIs back to the original settings, which means one can upload FCPX to YouTube like they did before. Just expect Google to leave things alone for a longer while!
However, my personal preference and recommendation is to handle sharing videos from FCPX to YouTube in 3 steps:
1. Export a Master File: "File > Share > Master file". This could be the fastest way to export files with the highest quality.
2. View the Master File to make sure it looks and plays great.
3. Compress, or convert the Master File: the Master File needs using a Compressor or a Mac Video Converter.
Of course, this process requires a few more extra steps, but I'm quite sure that the results will be consistently with higher quality and most reliable. It is also the fastest way to upload FCPX to YouTube.
Big Note: Most video files are far too big to upload to YouTube without some compression. Compressor as well as the FCPX to YouTube Converter has an optimized YouTube setting which reduces the size of the file while minimizing compression loss. I think it is worth taking time to learn how to use the Compressor due to much faster encodes, higher quality and more flexibility in compression settings. But if you don't have the time to learn something new, then I strongly recommend you to download this FCPX Video Converter since it only takes 3 simply steps to convert any FCP video to YouTube with the same effect as using the Compressor and it needs no time to learn how to use it: