What Strategy Tester doesn't tell you

Hi, I’m newbie to MT4. I have made a few EAs just to get myself familiar with the platform.
When I tested them on the Strategy tester, all of them seem to come out pretty nicely.
I’ve seen people talking about the tester being not reliable. I can understand the slipage can be a factor when you trade live. Apart from that, what else there is to affect the efectiveness of tick by tick back testing?
Hope someone can shed some light on this.
Rwgards
Jing

I would suggest against mt4 and instead use ctrader then download ctrader algo then download break out candles