the real problem with these studies is the critics. If it's a study that says veganism is good, then everyone does that or has a moral imperative for that to be true will come out and say "what more do those evil meat eaters want?" while simultaneously the meat eaters will lambaste the study as worthless on any point they can find.
Of course, the situation works in reverse. The problem is that it's incredibly difficult to find genuine, unbiased, nuanced criticism of diet research. Some people are dumb enough to see research that actively admits to funding from some food industry and immediately they say "this study is total bullshit" which is just not how science works.