Well, let me start by saying the obvious, I don't think you'll get a gigantic crowd of people swarming this thread anytime soon. If Serebii fan-sprites was dead 3 years ago, it now has the booming activity of J. Stalin's grave.
That said, critiquing each one would take a lot of time, and be mostly pointless when you've made your own style. SO, that said, the biggest flaw here that I see as whole, would be the consistency of both design, and spriting. The fire/steel Oni line, is without a doubt the standout line here in terms of both, so I'll compare others to it. While the whole line(especially the last), has a very clean minimalistic shading style, combined with hue shifting, clean linework, etc., you go to one like marqat, and the colors are flat, and many parts like the feather/hair, have every little strand shaded.
Now, most of them fit pretty well together, but a few marqat, navangler, and tyranuke are just a bit too complicated in terms of design, or spriting, to match with the rest.