In Luton, two hours waiting for customs to arrive (at 3AM), in Stanstead, they wouldn't let us take soup through to the aircraft (outbound), In Nice, an insufferable SAFA check, in China, I had a visa refused, in Riyadh, a four hour delay to insure we really were approved to depart.
Frank, the wait in Luton was unusual and shouldn't have happened, might have been expected as the airport was out of hours. The other issues don't relate to customs at all.
Customs in USA is like a lotto. I've had my best experiences in MIA and TEB, and also my worst experiences in the same exact places. Despite the CBP "pledge" advertisement places at all ports of entry, most (but by no means all) of their staff have serious attitude problems.