I just discovered this mod and I wonder if it is compatible with Embeddium (Rubidium) Extra

At the moment is. But it has some features than are implemented on Emb++ (like the FPS display, Cloud Height and fog toggle)

And I said at the moment because I will implement all Sodium Extra features on Emb++ (and Sodium Extra is the mod from comes Rubidium Extra).

So is safe to use, but don't expect duplicated features of both mods working at 100% (mostly because they colide :P)