To me, it's the hard edges and the lighting. All of his works have very clear and precise lines combined with subtle, but very effective lighting.
And he's really good at choosing effective angles. The angle in this image makes the mermaid look very massive and impressive:

And in this work, the angle is just so very slightly tilted, giving it an uneasy feeling. Something is wrong, but you're not quite sure what it is.

Definitely a really unique style, ionomycin is one of my favorite artists.