Leaving aside the questions around the ethics of 996... is this kind of schedule actually effective? Do companies that work knowledge workers like this actually outperform competitors?
It was massively effective. I wrote a ton of great code. I made enough money to quit working for several years and do a startup. When you're young, moderately fit, and ambitious you can crank out a ton of code.
However, the important point is that it was my choice to do that. You can't ignore the ethics because that's what makes it wrong - asking people to do it while (even silently) implying bad things will happen if they don't is deeply unethical.
I can happily crank out code all day if it's something I'm doing for myself. As soon as you add the employer, it immediately starts doing my head in. It's completely different work. Some employers are better than others in this regard I suppose.
If there are a lot of people in the company doing 996 every day I think they're probably spending most of their time doing mindless busywork.
It is not like you can do the same project twice, one crunching and one not and see what the results are?
In terms of health, definitely not good.
Also from economics, makes no sense if you are not a founder.