The only way to know is to bench it yourself on your project since the overall scene complexity matters.
as far as I know and anyone can tell, working material shaders to shift vertices is much less expensive and way faster then it is to use skeletal meshes and animations.
meaning if you have a crowd of 1000 people, each with full skeletal rig, you get very poor performance.
if instead you convert the animations to some sort of vertex shader/movement you can still have very good performance.
Blatant Example of this would be Auzu’s fish schools. Look up their videos for talks on it and on how they do it.
Keep in mind its not Exactly the same as a morph target - though it is in essence. You manipulate the vertices position GPU side.
What you would need to bench however is the actual cost of shifting the morph value from 0 to 1 on multiple different characters at once.
Each would have to be imported as a differently named thing in order to prevent automatic instancing.
I would start with 1 model doing something simple like a blink, import it 20 times with different skeletons, and coding the level to shift all 20 morphs, either at once or though a for each loop.
With that you can sort of find out what the cost is, specifically for your project, and determine the radius in which you will be animating multiple morphs.
iits most definitely a lot of work.