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Drawback to promotions?

Is there any drawback or danger to promoting an unknown member who is a great contributor?

No, there is no drawback. The guild leader can demote them back down at any time if need be.
I should add, however, that I would get to know someone before advancing them very high in rank. For instance- at rank 2 they have the power to promote. If they went nuts and started promoting everyone then the guild leader would have to reset everyone in the order that they should be in.