Shape Keys:
what is worse and was not as obvious as the "hidden" UE importer options:
You will get rid of your shape-keys when you select the apply modifiers box in blender fbx-export.
This is not a special fbx feature, it happens for all types of export I meanwhile noticed, And in a way the reason is simple: A shape-keys is a second mesh inside blender (not joking, all vertices are copied, even when you change a single vertex only). Applying only works on one layer.
These are the exporters:
https://docs.blender.org/api/current/bpy.ops.export_scene.html when looking carefully, for bpy.ops.export_scene.fbx, option "use_mesh_modifiers" you will notice:
"Apply modifiers to mesh objects (except Armature ones) - WARNING: prevents exporting shape keys"Be aware that blender shape-keys can be created from -10 to 10 (afaik) but Unreal accepts only -1 to 1. So they must be normalized, when they are bigger than 1. In this case: if you use a bone driver in blender and you create the animations you need to adapt it to the other norm also.
Materials usually work rather good, as long as the principled node is used. For eyes I use two materials, so a glass layer to reflect light. Of course this only makes sense when the face can be shown from a near distance, but it looks rather good.
I use own animations and standard skeleton (no toes). It works as well, but I can't use pre-made animations. On the other hand, my model can talk without shapekeys and without modifications after I created the 11 phonemes with a poselib.