This is of course a very hard question without a simple answer. So we can just theorize.

What you call one-size-fits-all approach, I could call a perfect strategy that stands the test of time. If I had one that trades on BarPeriod less than 4h.

The other approach would be to optimize for the recent market condition, and then as soon as the strategy becomes unprofitable, drop it and replace it with another one. This looks so good on paper, until you realize you would need a proper and fast algorithm for that, and of course, that's a problem in itself. You would also need lots of predefined strategies. This approach gets very complex very fast, although I haven't completely lost faith in it.

Actually, I started some research with this second approach some weeks ago, but stumbled into a Zorro bug, so it'll have to wait until Zorro is fixed. That was also my pet project to see what equity curve trading is good for.

There was an article on mechanicalforex site where Daniel explained why he thinks that you should always backtest on at least 10 years of data. But, I can't find it anymore, I'm afraid you'll have to dig yourself. While we all have our own opinions, I tend to mostly agree that a strategy that is profitable in the longer term is better, because there's a greater chance that it will continue to be profitable in the future.