Considering how Unity’s ECS is a completely alien entity (ha!) to GameObjects and MonoBehavior components, yeah, an “Unreal ECS” too would have little to do with the GameFramework to get 100% efficiency. You could reduce the overhead of dealing with individual StaticMesh representations of your entities by using ISMs, HISMs, or even feeding the data into Niagara.
“What about animated characters”? AFAIK Unity doesn’t quite have that yet. There’s a package that can bake animation into textures and materials that can play those back, for instanced skinned characters but it’s far more limited than Mecanim due to the nature of the technique.
BTW I have plans to code an “hybrid ECS-ish” framework for some upcoming projects, but the use case is to serve as a basis for rollback netcode, so it would still piggyback on the GameFramework to display the simulation results because the goal is being able to run the simulation 8x per frame (to support rollback) but only “display” it once.