Rigify puts in a lot of extra unnecessary bones right in the middle of the deform bone hierarchy. This means you can not export just the deform bones without exporting a lot of useless mechanism bones that don’t do anything in the engine. The problem is specially compounded if you use rigify’s facial rig that has 87 deform bone before you start counting all the junk bones.
I made a script for Blender 2.8 that deals with all of these problems and offers a simplified way to get rigify working with UE4. As mzprox mentioned it is a paid product but do check it out anyway.
