They often act in bigotry, fighting those who say negatively about them.
Also while other religions are created by people with epiphanies or someone who demonstrates divine powers etc., Scientology was created by L Ron Hubber. This wouldn't be too much of a problem except that Hubber was a science-fiction writer and much of the stuff Scientologists worship is sci-fi-ish. i.e. To most it seems more like a cult than an actual religion.
Its also the STUPIDEST religion based on a crazy belief. Also apparently when you join they take you to a log abin in the woods and hook you up to a lie detector and find out all of your dark secrets- so you cant badmouth the religion or theyll tell everyone everything about you.