The advantage is arriving at your US destination as a "domestic" flight.

The disadvantages depend upon how the clearance is set up at the point of departure. We have used pre-clearance at Dublin and Aruba.

Aruba involves several steps duplicated by local and by US officials including checked luggage that must be reclaimed and re-checked a second time, removal of shoes, belts, etc., twice - the entire process can sometimes take up to two hours but often is less. There is no "express" service on Aruba.