I don't know if I would say "every" set is a good seller. Not a single Hidden Side set is backordered. Several are even on sale at this time and still not sold out. Hidden Side Train has been 25% off for over a week at Target and you can still buy qty of 10 for shipping (https://www.target.com/p/lego-hidden-side-ghost-train-express-70424-augmented-reality-ar-toy-train-building-kit-697pc/-/A-75667055).
Hidden Side reminds me of a sports study done in the USA many years ago trying to determine why inner city kids typically do not play baseball or hockey. It was determined it was the cost of equipment. Inner city kids were more likely to play basketball or soccer because all you needed was one kid with a ball and a hoop or target (goal).
Hidden Side sets are nice, but unless your kids have access to devices, they're not able to play with it to its fullest. For those parents (like myself) whose kids already spend too much time on electronics, LEGO was supposed to be the escape from electronics. I have to admit, the videos advertising the Hidden Side actually turn me off. A group of kids standing around a LEGO set playing electronic devices? Kids are already doing that. Why would I need to buy a slightly more expensive LEGO set sitting in the middle of them not being played with?