The APD tax is certainly a disadvantage, being the only region of the UK with a land border with another European legal jurisdiction.
The only disadvantage is to US carriers at BFS.
People who want to go to the UK are not going to go to DUB just to save a few pounds. Before the APD was introduced it was not as if BFS was well connected in Europe and as airlines didn't start operations then why would they now when most are cutting back. If the tax was scrapped it would help the current carriers but if it stays or goes I don't see any European carriers starting at BFS regardless of the Tax. Why would they now when people are traveling less and less.
BFS like most airports in Ireland becomes to dependant on Bucket and Spade routes over the last few years.