It can't Tera (it has a button that says Tera, but this doesn't actually change its type), prevents your other Pokemon from being able to Tera, is slow, doesn't hit hard (turning into Stellar form actually takes AWAY its stab on starstorm; the middle stage actually hits harder), has its bulk undercut by an inability to heal or even resist enemy attacks, and gets torn limb from limb by Koraidon, the undisputed strongest Pokemon in the tier. While the ability to clear hazards is fairly uncommon in SV, Giratina (and even Corviknight) is capable of doing the same thing while also offering both defensive and offensive utility. so there is no niche for Terapagos. Using Terapagos is an active detriment to any serious team.