If you had been dedicated enough since release, then ROI would surely be high. But if you have invested right before the peak of 2017's bubble, you would have lost more than you could have gained.
Basically, investing regular amounts on regular intervals is what cost-averaging is all about. It does lessen the risk of losses and has high probability of gaining, but it still depends on the time and price trend upon your entry.
This is not entirely true, you never lose unless you sell. The problem is, are you willing to wait whatever it takes? Because if you invest money that can be "lost" or "frozen", ie. long term investment, you probably has no issues waiting years or decades. Of course that only works with your own money, not talking about margin trading.
Buying a little bit everyday simply spread things out. Long term bitcoin price goes up, so yes it should work fine. Even if you bought a few coins at nearly 19k most would have been bought 3k~12k prices the following years anyway.
If you are in for the long term, the price is nearly irrelevant. Bitcoin excels for long term savings. Fluctuations beyond its "natural" market price don't last long anyway, so most the time you would be buying at fair price, sometimes you will get expensive but also sometimes cheap, i guess they average in the end.