There's a dedicated person doing the Unity runtime, while the Godot person also supports multiple other runtimes. As a result, you can get your Unity bug fixed within a working day, while Godot's bug response times take considerably longer for understandable reasons. Unity is just a more popular and more mature platform in general.
By adopting Godot, you also have to deal with worse support and integrations for all tools, not just Spine. It's a rather new engine with considerably smaller user base. We're already seeing increased demand for Godot support here and elsewhere, but that takes time to ramp up. You're basically an early adopter with all that entails.