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.

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