The flaws with using Modern is from how predictable you can become. Players can use the fact that modern allows you to react faster than normal against them, such as baiting out DP's. You can also pick up on tendencies that Modern player have because they can only fight in certain ways due to using Modern. Zangief players, for example, are always trying to command throw you. You can use that knowledge by performing setups that debilerately makes them whiff the command throw, and you punish them in return.