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Many mega evolutions will not be placed in ubers as they are just not good enough. Others may be suspected by our normal process. It's unknown how exactly this might work, but it's expected that if a mega evolution is found to be broken, its mega stone will be banned, not the base pokemon. In the future, though, you should ask questions like these in our simple questions simple answers threads (but be sure to read the rules); this area of the forums is for competitive analysis writing.