Yes the Mt5 strategy tester is reliable. If you are having issues with back tested results matching real time trades, it will be in how you have written the code and system design.
The data you test with comes from your broker, the data becomes less reliable going back longer than 5 years. You cannot buy historical data and import it into Mt5, unfortunately.
Use the “Every Tick” setting or you will find the algorithm used in the testing will approximate market price data input, this is to shorten the time taken to analyse the data. This result means you will get a distortion between backtested results and real-life trades. A complete waste of time.
There is no short cut to testing several input parameters, it does take days to complete. Depending on how you have written the code it can take longer.
With our testing we focused on testing individual parameters but within those individual parameters we have written code to give us range of optimised setting for each input.
The current EUR/USD EA we have given out free for people to use took over 2 years to design, code and test.