Personally, I create the basic assets into a folder, then draw from that folder for each Editor/Engine I sell for. All assets are identical in both Unreal, Unity etc etc… with the exception for the Materials, which are bespoke.
When I create a pack that is just Meshes and Textures, I do so from the exact same source folder, so when you buy an asset pack from our studio, you are getting the same assets, just the engine/shaders/materials are different. I do not believe in reinventing the wheel each time.
One thing about the Generic packs that are not created with any Engine/Platform/Editor in mind, is the purchaser must create materials etc themselves.
The Meshes are optimised for almost any usage.