The main problem for me is 3d meshes.
When it comes to 2d assets, we kind of have a few standards people use and generally loading them is trivial.
However the same is not true of 3d assets. People use so many file formats, and not all support all the things you want.
I regularly have to re-texture objects because I find UV coordinates out of range, generate maps for this or that, fix problems.
Importing a 3d asset should be a ten minute job, but it is often a two day job.
Wish I had my own graphics artist