Weird mesh after fbx import from Blender

I'm having a trouble with an animated model exported from Blender in .fbx format into Unity.
The model has shape keys drivers that better deforms the mesh along with the bones and I don't see any problems with them in Unity.
The model, when I animate or pose into Blender, is perfectly fine and all the joints meshes are ok, and also if I import the mesh into Unity without the armature, in a pose, joints are ok too. The problem is when the model has an armature in Unity, some joints are distort and different from the model in Blender.
I don't think the cause can be the 4 bones limit that Unity has because I tried to limit that number in Blender and the result is identical. I know it's a little difference but after all the work to adjust the bends I would like to avoid this glitches.
Here I post comparisons between some of the result in Unity with an armature(first pictures) and without and armature(second pictures).
Hope someone can help me