H5m (Hook5 Mesh) is not a format that can be used outside of Hook5. I think comparing it to FBX, Collada, or OBJ which are all text base is comparing Apples and Oranges. To use it with Unreal or anything else it would need to be read and then converted to a format Unreal can process.
You cannot import a H5 Mesh into substance painter or any other program for texturing. You have to use a universal format Substance Painter can read (OBJ, FBX, or Collada). So if you do not have the original mesh, you would need to convert it using CTK. While H5M supports dds and PNG, it is important to know it only supports both for the diffuse file. PBR textures must be in dds format.
To your point...like Unreal Assets, H5m is a proprietary format designed to work specifically for H5, which is why it is efficient. But it is not universal so anything you want to bring in will need a loader or conversion process.