The quality of animation has nothing to do with the software.
It's not like once you switched softwares the models magically start moving smoother, it's about understanding the principles of animation (blocking, stretching, timing, weight etc.) and the how the shots are framed, meaning the relation of the positions of the models to it's environment, determining the focus of the shot and so on.
Think of 2D-Animation. The oldschool stuff from the 80-90ies absolutely passed the test of time and I'd argue most modern anime don't even come close.
The "big" disadvantage of SFM for me is the workflow. I don't want to get too technical about it but the process of making adjustments in Blender and transfering the models from software a to b is fat pain in the ass because you'll be doing it a million times, that is if you wish to be somewhat independant from the assets being provided on websites like SFMlab and SFM's workshop.
Then you have the technical limitations being the amounts of polygons SFM can handle (you want to limit them to have shorter render times anyway tho), the lighting looking janky and that it doesn't use the potential of 64-bit systems because of it being capped to 32-bits.
When it comes to "look", there seems to be common pattern with Blender artists that their content has a squeeky clean barbie look, which is obviously not something I'd go for. There are definitely exceptions such as ZMSFM and MendezSFM, so I'm assuming it's not necessarily because of the software either.