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1. Falls to fighting, ground, and special attack moves easily
2. Only decent STAB moves are Iron Head, (which it's too slow to use to flinch), Sucker Punch (which is too situational), and Night Slash (which is pretty weak)
3. They're not slow but they're also not fast enough to outpseed most mons.
Basically they are really good but only if used right. Also it makes Aegislash his bitch.