GREEEAAATT!!!
I've just solved the MIXAMO characters/animations importing.
It was due, as I thought, to the "more than 4 bones per vertex" problem.
JUST SOLVED!!!
Now characters support up to 8 bones per vertex that it is more than sufficient.
I've also left the "4 bones per vertex" shaders for using them in case the skin hasn't vertices with more than 4 bones (faster)
The following is the elf model taken from FUSE and them from my MIXAMO account. I have rotated some of nodes to show they are correctly skinned.
This is only one of the improvements you will find into next update
STAY TUNED GUYS! next update coming soon :D