Does telling a girl that you like her backfire? If so why?

Some say if you tell a girl how you feel that you blow it. I only think you blow it if you don't know her well and just say it. You put her on the spot and she doesn't know what to say. If there's already an established relationship then I don't think expressing how you feel is bad.
Does telling a girl that you like her backfire? If so why?
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