The downside to clipless I've found is when your stable platform is of limited real-estate and you have to make the choice between you and your bike. With platform pedals, you can simply discard the bike to one side or another. With clipless, it's a little harder as you have to first remove yourself from the bike which can most often use up that precious real-estate or there's simply no wiggle-room from which to work with in order to execute a dismount.