Ok so lets begin:
The signature is good. I like the fact that apart black and white, you focused on not more than 2 colors. Good. Yellow and light blue mix good.
The problem starts in the stroke ( I mean, the edges are too pixelated, something that could be solved by smudging slowly on the outline, and paste a duplicate of the original render on top, or simply by selecting the outline, feathering like 5-10 pixels, and cuting the excess.) Much better that way.
The sphere on the bottom right corner feels a bit out of context, since it's too bright and it catches a lot of attention from the main render in the middle.
If I'm not mistaken, you placed a light blue layer on top and mixed with the rest on Hard Light settings rigth? I think that's the main reason the pixels are way too visible.
Overall it's a great work, so I think it deserves a rate of 8/10 at least. Pleasant color scheme helps a lot.
Keep it up.