I'd be quite worried about breaking it in the not unlikely event of a crash, additionally it looks like it could quite significantly limit how low you could put your seat down on a mountain bike, so that would get in the way. Even if you got your seat down, it would add a significant amount of height above the tyre so you mighn't be able to move your arse far enough back to shift your weight property. It's also adding nearly 4kg weight relatively high up which probably isn't the end of the world but would be better if it wasn't so high up.

It says on the website it will fit hardtails but not full sus, and that it isn't recommended on knobby tyres due to performance which isn't ideal either.

I could be wrong on all of this but those are just the concerns coming to mind as another mountain biker.