It has excellent stats, and a good typing. Specifically, it's immune to common moves such as Core Enforcer, Spectral Thief, Ghost Judgment, and Moonghiest beam, and the only relevant move that hits it super effectively is Sunsteel Strike--in addition to a great resistance to Knock Off. Thus, it is difficult to take advantage of, and can run sets like Fur Coat to be a good physical wall and the best Mega Gengar check out there, or run a Water Absorb set and not fear switching in on Core Enforcer, or even just become a difficult-to-break status absorber with Poison Heal.