They are easily stressed during shipping. This makes them shy/nervous in a new tank which often results in refusing food, so they can be difficult to start feeding. They almost never eat the entire time from collection, transit, through wholesaler tanks and when in retailer tanks (the latter may be able to get a feeding response in some cases). Thus, they very often arrive to hobbyists highly stressed and in sub-optimal health from not eating for 1-2 weeks. Large angelfish are also known to have high rates of infections by flukes and may pick up other parasites/diseases as they move trough the supply chain. Unless the hobbyist has the right conditions such as a quiet QT tank with plenty of shelter spots, good filtration and ready to treat for diseases, most regals don't make it.
Once healthy and eating a variety of foods and in an established tank, regals are can be quite hardy and bold. Mine push their way to the algae clip with all the other big angels and tangs. It's getting them to that point that makes them a species that novice hobbyists should avoid.