I think the CR lower than HD - and IME a pretty fair CR - says a lot, and I'm pretty sure that anything above +0 is pretty hard to justify. You lose a point of base attack bonus and four levels of class features, as well as practically all your skill points, and in return you get some strength and con and are bigger (though being bigger isn't strictly beneficial).

(Also, am I the only one who thinks that the artist's signature looks like "Fish", followed by a skeletal fish, and is wondering just how deliberate or otherwise that is?)