The fact that it's manually traded (yet, it really is) adds a human intuition element that MDP doesn't have. If the swings in the market get larger, they can adjust for it. If the swings get smaller, they can adjust for it. I think the one issue that could cause the strategy to fail is if the market starts to trend more often than it ranges. This is definitely a strategy designed for ranging markets...
The other, 'main' issue, is human emotion. They're trading almost around the clock. That takes a lot of focus, effort and coordination. They're getting paid very well for it but it can take a toll on the body and emotions!
Adding transparency and quality to the Forex market, one trade at a time.