I ran a single contract ES strat for 2 weeks on paper and then tried an actual account. Day 1: 2 errors. Day 2: 4 errors, Day 3: 7 errors. Errors were things like fills that were anywhere from ~12 seconds to ~4 minutes late. But the killer was the orders were so late, I already generated close orders. Then all sorts of chaos ensued: Positions closed, position double opened (overfills), Positions double closed (overfill and reversed). The syncc engine has to make a judgement on when it is out of syncc and WHEN to get back into syncc. You cannot try to out smart it because it is a black box.
Needless to say, I pulled the plug after that. I chatted with the guys at C2 and they were very helpful and transparent. Like Snuskpelle says, it works for swing type trading but not day trading where you hold for less than 10-15 minutes.
So regardless of the Sim or any other data, when the rubber hits the road and you actually trade, then you will uncover all the limitations.