Humanoid Rig causes issues in animations

Hi,
I ran into an issue with importing a animated character (.fbx) from blender 2.92 into unity when changing the rig to humanoid.

For testing, i created a simple pose in blender holding a firearm:


i exported my model and rig with the following settings:



Once imported to unity everything looks fine with the generic rig:


until i change the rig to humanoid in the import settings:
The fingers rotation, especially the right hands thumb is very far off


Looking for the avatar configuration all bones are showing up in green and correct order. I do not get any error messages.

The model is created with VRoid Studio, so its rigged and weight painted automatically. Could that be causing inaccurate animations? Do i have to redo the rig? Are there any specific rules for mecanim to recognise a humanoid rig properly?

I have no idea where to start looking for ideas what might be the cause. All resources i found didnt do anything different from what i did. I am 100% lost on this so any hint on a possible fix is appreciated.

 

Reimporting the fbx to blender works fine btw. so its most likely unity that fu... fails up my animations

 

Ok, so thanks to JayAnAm
Blender rig for Unity | JayAnAm - Tutorials & 3D models
i had some reference to fix bone roll what helped quite a lot in animation accuracy but isn't 100% there yet.

Think i kinda have a idea what mecanim expects my model/rig to look like to retarget properly. Bone roll and position of joints relative to the mesh matter way more than expected. Frustrating how unprecise the unity docs is about this.