The problem with looking at demo's and then trading reality is the cost and slippage which are often more than you may imagine, particualrly when you account for the potential for complete screw ups ocassionally and fast or thin markets producing large slippage.
One demonstration I have used many times is to show an equity curve of a system and then progressively add 1tick slippage to it per order and see how quickly it degrades. This is the problem when you move from trading 10's or 100's of thousands into trading 100+million.
In my early days I worked for a London fund and we built up a big oats position, upon exit we sent it limit down 2 days running, still see it on the chart now. That was a lesson.
I more recently worked for a multi bln fund focussed on metals. The effect that their positioning would have on flat price was substantial and certainly far greater than most algo systems could sustain. The interesting point here was that their orders would tend to trigger the algos in their direction and it seems there can come a point where throwing huge volume behind and order can actually create a cascade where you benefit. For most of us I think that execution is where the money is made and lost