GVA mostly gets a pass around here, even though he was involved in a doping investigation. He was acquitted, though ....by his national federation.Sagan in general gets far less attention from the Clinic than you would expect given his performance levels and some of the ridiculous stuff he's pulled over the years. The guy clearly has underlying incredible talent, but he's pulled some performances over the years that, if they'd been done by somebody who people weren't willing to turn a blind eye to because he pulls the occasional wheelie or dyes his goatee green, would have been absolutely pilloried, if not in the same way as Cancellara or Valverde then at least in the same way as Gilbert or van Avermaet.
Sagan had one of his worst seasons since he turned pro, but it seems to be mostly a motivational issue, as he was still among the very best in the WC road race, for example. Valverde, GVA, Gilbert and Cancellara have more questionable results / performed at other levels than before, whereas Sagan has mostly been consistent, but added muscle/weight and lost explosiveness and climbing ability along the way.
He's most likely been on an elite doping program for all his career, OR is incredibly talented and is only on a run-of-the-mill program. It just isn't as in your face as with Cancellara and them.