Without a stable and widely used model format making a game is impossible. You can drag&drop FBX into Unity and Unreal, and its mostly ready for use. Having a broken FBX importer is just a waste of time. You start with a model format, something works, you build a workflow, a system for producing content en masse, and then at some point you must consider throwing away the model format, the workflow and/or the whole engine, because the importer is unreliable. Or you try to rewrite it yourself, further wasting time. Some things must be rock-solid, open-source or not.