I've been doing Market Replay for NQ/MNQ and ES/MES for about the past 8 months and have found I'm about 70% successful in trading. I don't trade every day. It's about 3 days a week on average.
Average winning on MNQ is about $25 a day (if I divide by days traded).
that said, this has only been in Market replay (I download the data in NInjatrader and use market replay).
Is this a reliable way of testing a strategy?

ImActuallyStupid 

Is this a reliable way of testing a strategy?

Hard to say. Are you scalping for very small profits on lots of trades where your potential profit could be affected by optimistic sim fills for instance?
Are you replaying the day in the evening so have a vague idea of what happened during the session before you start the replay, eg, the day ended higher?

Also, a question that comes to mind is that 8 months of replay is a lot, what about results when trading on sim for example, but on live data, ie, in real-time?
And if there are no real-time results and you are not actually available to trade in at those times, hence the long term practice later on replay, what's the point of practising trading a session you are not currently able to trade, perhaps because of a job.

Lastly, if you have no real-time trades using actual money rather than sim, expect those results to be not be as good as the sim results, at least initially, due to the psychological side of having real money on the line.

Just a couple of thoughts from reading your post.