I'm far (very far) from an expert on snow unicorns, but I will say that riding directly over the shoulders just sounds...unpleasant. There's a lot of movement there, and it's the highest (and narrowest, I think) point on the spine. Most critters are ridden with the rider behind the shoulders (horses, camels, etc), with a few having the rider sit in front of the shoulders on the neck (elephants). Not saying you're wrong at all, just basing my critique on the critters I do know.

Regardless of rider position, the withers (high point of the spine/shoulder) don't quite line up with the forelegs on the foreground unicorn.