Yes Merkx failed a few tests. The difference between then and now is everyone was on pretty much the same stuff and it just helped riders through the stages, but the best riders still stayed at the top.
Once blood doping came in, you can alter body values and also rans could boost themselves to winners.
The UCI set an EPO / hermatacrit level of 40. If rider a) had a natural level of 33 and rider b) had a natural level of 25 they had a bigger performance gain
Even genetic freaks like Miguel Indurain and Marco Pantani were juiced up to tne eyeballs in the 90's - and Noughties. It was endemic.
