Buildings with zero occupancy (but small loans) are not really a problem . . . that is just a rich guy buying stupid shit and happens all over with boats, planes, travel, toys, etc. The downside there is a the reduced wealth effect as the person who thought they were wealthy spends less.
Buildings that are worth less than the loans on them is a real problem. Building owners default, banks stuck with bad loans, banks go bankrupt, government bail-outs, currency deflation, etc . . . a whole chain of disaster happens.