I think it's pretty great, if its ability is Wonder Guard, the concept makes a whole lot of sense. They said the crystal can't be damaged and if it gets Wonder Guard the only chances of it getting hit by an attack is Fairy.
Also, it goes with Sableye's overall concept. It's a creepy-lookin devil thing but it's actually a scared and innocent Pokemon. Sableye hiding behind the crystal that has been embedded onto it since birth is kind of like Flabebe's line.