I see absolutely no downfall to that and it satisfies the original question, which I would guess, since being on “Unreal Engine” forums, refers to good “Unreal Engine” games using free “Unreal Engine” assets.
It’s only normal that Epic would want their assets to not be used outside their engine, given how vastly different Epic’s monetization strategy is to other engine developers, say Unity. So yeah, if you want to use free assets from Epic’s Marketplace, maintained by Epic, kept safe by Epic, distributed by Epic, I think it’s only fair use towards Epic that we don’t use their provided assets someplace else.
You can use the .fbx files in any engine after extracting through Unreal Engine. You can use most base assets anywhere after extraction, such as textures, static meshes, skeletal meshes, sound files, etc. For them, it’s a matter of morals. I doubt, even though they said not to, the creator or Epic would pursue you if you used a sound file provided on their marketplace, in Unity.
It’s the complex assets like particle systems and blueprints that can’t be migrated since they’re built to the engine’s specification and at that point it’s more a matter of keeping them working properly with the engine than to prevent users from migrating them.