They shorten games with their incredibly slow tempo and that makes upsets against them more likely. I know that style of play is their identity and all, but creating more possessions almost always favors the better team just as playing more games in a series is advantageous to the better team in a playoff. An underdog is much, much more likely to pull off an upset in a one-off scenario like the MLB wildcard game or any football playoff (though favorites win more often in football than other sports due to the nature of the sport).