Thanks for this detailed report!
We just started a thread about Blender animation here to ask users to test it and give feedbacks. You can post yours there, or anything you have about Blender support It will be very helpful for us!
To answer about your points:
- the modifier "Subdivision surface" is not working with my animated .blend files. With static ones there's no problem.
Yeah, if your mesh has shapekeys, modifier cannot be applied to it. On Sketchfab we need to perform this operation in our processing. We don't handle this for now, but it is logged on our side. Maybe we will be able to improve this
-The weird effect with the beard in the animation is happening with other animations with shape keys (morphing), in my last test the effect was visible in the whole object. Static .blend files are affected too.
- static .blend files with shape keys in use have the same weird effect, but if those are all 0.0 (disabled) the model shows properly.
This effect is due to a normal generation issue on morph targets. Also logged we are working on it.
- The object constraint "Child of" is not working with bones, but it works fine if the target is an object.
I hope it can help in some way.
Constraints should be supported, but there are certainly some unhandled cases. Thanks for this, I note it!
Thanks a lot for these tests!