which markets is the question

See, I think a flat tax probably is more efficient money-wise (the article didn't show any evidence it). Just like a kidney market is efficient.

However in both cases something is lost. The efficiency comes at a price.

For kidney markets it would entail devaluating the body ("if kidneys cost $X, how much for the rest, sir?"). Quite a few people think it's worth it. They don't think bodies themselves are all that valuable enough.

For a flat tax, it would trade compassion/equitability for higher growth (more so than today, at least). It may be a trade society wants to do. Hard to tell. Either way, I think an economist stops being just an economist when they advocate for markets - it's like a bus driver arguing for more mass transort, or a nephrologist arguing for more kidney transplants. That's why so many economic theories are mere proxies for ethical, political ones.