Actually, white boxers can be registered with the AKC, assuming the parents are AKC or other recognized kennel club registered. Having more than 1/3 white is simply a disqualifying fault. It's right there in the standard.
In general white(or mostly white) dogs are more likely to have hearing problems, because the pigment cells that form melanin are also involved in important parts of the inner ear. White dogs have hardly any pigment cells, so much higher chance of not developing normal hearing.
Double merle pups have similar problems, only much worse. They generally have eye/ear problems among other very serious health problems as a direct result of the merle-merle gene inheritance. Sometimes the pups are born with no eyes at all. Many die in utero, or as pups and never make it to adulthood. Those that do survive are usually blind and/or deaf. That is why it is considered ethical to breed two merle dogs together. Most responsible breeders breed merle to solid, so avoid problems.