Uploading to You Tube seems to have issues out of iMovie iOS.
Exporting to the camera roll and sending to You Tube from there seems to work.
You should log the issue with Apple:
There will be no difference in quality from the camera roll versus direct export from iMovie.
Either way it needs to render out to an HD H.264 file, and both are the same file.
If you export to camera roll first , then choose to send to You Tube direct from iMovie it will just send the same file it used to send to camera roll.
If you export to You Tube from iMovie, then send to camera roll it saves the video it encoded for You Tube to the camera roll.
What I am saying is that it is exactly the same video/render both times.
There is no special render quality for You Tube.
You Tube though will re-encode the video in a lossy format again, and lose quality.
Playing on You tube versus playing a file form the Camera roll will look different, with the Camera roll video being the better version; but you'd have to look very closely to spot it.
You do lose quality editing video, as it needs to be re-encoded and the i frames are re-sequenced in iMovie iOS.
But again the loss of quality is minimal and you have to really know what you're looking for to see it.