What I mainly think you need help with when it comes to your cartoon kind of art (the spider thing and your purple one), is your blending of colors and placement of lines. The black likes look randomly placed on your spider between the eyes. Eyes seem pretty big too. A lot of the bolder lines look like brush strokes and that's it. No touch ups or anything.
With your purple, demon? I wanna say. Idk what it is. As with the spider, it looks like simple brush strokes where you're adding lighting and shading. You should work on your contrast. Use the smudge tool and blur tool.
I can't criticize your robots, since I am awful at robotic textures. Mainly because I haven't tried before. With those textures though, it's the brush strokes for shading and what not. That seems to be your biggest flaw when art'n. Also, may want to clean up those dark lines that come off the base of the shapes that stray from the actual outline if you're going for realism.
TL ; DR
-Watch your brush strokes. Touch up on them so they aren't just, brush strokes.
-Make sure to blend your colors more for shading and lighting.
-Use the blur and smudge tool.
-Clean up after you're done for minor details.