The head looks completely detached here.

Flagger said:

Bad anatomy (detached head -> reference photo)

Provence said:

The head looks completely detached here.

The artist has a response for both of you.
http://lvltheperv.tumblr.com/post/161078754935/i-would-like-to-break-spines-to-show-the-part

Apollyon said:

The artist has a response for both of you.
http://lvltheperv.tumblr.com/post/161078754935/i-would-like-to-break-spines-to-show-the-part

The reference photo seems to show what this post is missing: This has nothing to do with the ass, but it looks more that her head is floating somewhere in the air. Just look at the placement of her left shoulder (or where it should be). Then the placement of her head makes no sense.

The ass to back connection is absolutely no problem.

Have to side with the flagger here. If you compare the picture to the pose reference he gave, you'll see that the torso has to bend forward in order for the head to end up in that position, but here it looks a bit too erect to be plausible. (Possibly the fault of shading gone awry.) That's on top of the art and subject matter being borderline off-topic in the first place.

Hmm, most of the upper part of the torso is covered by her hair and shoulder so I wouldn't say with confidence that the torso is more erect than you're suggesting.

Squishy said:

Hmm, most of the upper part of the torso is covered by her hair and shoulder so I wouldn't say with confidence that the torso is more erect than you're suggesting.

The curvature of her spine is drawn quite clearly on her back, which is why I suggested the shading being at fault.