To the people who are 'scalping' with no problem, I seem to remember they define some type of smallest allowable hold time for trades. For people doing manual 'scalping' this would be no problem but it may be for automated systems. If you want to read about their famous 'price spikes' just Google around, it won't take long to find a story, but of course it will never happen to you As with all things, it really depends on what you want to do, no broker is perfect, for most conventional styles of trading I'm sure Oanda is fine.
The breaking of a wave cannot explain the whole sea.