if your strategy heavily depends on ticks or calculation based on ticks (or not closed current candle) - opening and closing trades, than it is not possible to test with good accuracy in tester
if your strategy depends on repainted signals, it is not possible to test with good accuracy in tester
if your strategy (opening and closing trades) depends on history candles (first closed candle, or second, or indicators based on that, and so on) than you can test it in tester
.... and so on ... it all depends on that strategy
.... and also for demo/live, if you was profitable first month, the second one you can be lossy (this is true for all strategies) ... forex is more lossy than a casino (99% of all traders lose their money - if anyone claims the opposite, than is working for some organization (books, forex training, signals, EAs based on shits, broker itself) or is not experienced enough)