100% agree. Having more than like 3 skinned mesh renders should not be allowed. And if im not mistaken, there is already a limit as of writing. i just dont know what the exact number is.

Mesh is easy to merge, Though it is handy to have some meshes seperate while in Blender, but could easily merge and then export FBX. However Materials are a bit trickier. As some materials need to have seperate components to have different effects for different parts of the avatar, especially on avatars with attachments that wouldn't exactly work well. Yes some shaders have a Metal map, but gloss is not easily variable.

Having 20 materials is excessive, but I could see putting a limit of 5 materials, I think that's a fair amount, encourage people to try and merge materials into one. My phoenix uses several, but... I could merge maps, and just extend the texture from 1000x1000 to 2000x2000 and put all the textures into like a quadrant, or at least use my Tetris skills to fit all the parts.

(Edit 02JAN2018. Which as of now, I managed to get my phoenix down to just 4 materials, and my latest fox avatar only uses 2, which if I redo the original outfit, I should be able to get down to a single material.)

I personally think the limit for skinned mesh renderers should be quite strict, as long as it's clear as to why, and there is good documentation for combining meshes into one.

Materials could probably be more forgiving because it's a bit more complex joining up multiple textures and keeping them working right. That and often having more than one shader on different parts is required for some things.