I don't know, I might make a distinction between "reliability" and "build quality". The design of the slot loaders is at least "nice", while the tray loaders are an utter mess inside. (It seems pretty obvious the tray-loaders were designed *very quickly* to get something new-looking out the door as fast as possible.)
Even when it comes to reliability... I dunno. Tray loaders might hold up *slightly* better because they don't run quite as hot, but they still blow their flyback transformers at a depressing rate. I know the slot-loaders have problems with their plastic cracking but the tray-loaders are not immune to that either. Even when they were just five years old most of the examples I would see looked pretty shoddy. Just about every one had that cable door on the side broken off, for instance, and they suffer from poor fit-and-finish in the joints around the screen bezel and the back of the monitor shell.