A few years ago, the "Bad Science" books sold well. I think it's worth reviewing what their author, together with a colleague, said. One reason I'm impressed by it is that, as far as I know, neither author has "skin in the game" - their interest is not as cyclists, nor as front-line practitioners, but simply because helmets illustrate how complex and misleading the world of medical statistics can be. There was an
overview in road.cc, as well as
one author's own summary. In general, science works by overall consensus, not by quoting individual papers as "proving" things - only non-scientists do that. Rather, a single paper is a contribution to the whole, that is subject to comparison with and challenge by other work. So it's helpful that this paper is taking an overview.
That's not to say that there isn't significant evidence in favour of helmets, including favourable overviews of research to date, but equally there is negative research as well. As far as I recall the first doubting paper was in the
Journal of Product Liability around 1987. There was a reference up-thread to risk compensation being proposed as an argument against helmets, but that's not my recollection. That and several other points first arose in the debate precisely because research was showing that helmets were highly effective, and real-world experience wasn't, so explanations, not counter-arguments, were needed.