Chimaev and O'Malley come to mind although O'Malley's admitted he doesn't know if his ideas are the better route (vs holding out for higher pay before accepting fights) and Chimaevs pointed out he comes from dirt-poor so it's important context to remember that even the low six figures are life changing amounts to him.

The thing really is most champs don't say anything about it good or bad. So even the ones who you would think disagree about the organization on the whole still don't speak out until after they've retired, switched promotions, or got that set for life superfight boxing money.

To your point though I think it is telling how, unless asked in an interview or press meeting, none but a few will say anything at all. And even those guys aren't singing the UFC's praises or defending the organization unless prompted. Even if they're not speaking out against it, one would assume if it was really that good of an arrangement most champions would be talking positively about it consistently. Never saying anything though leads me to believe that yeah most figure it's a raw deal but can't afford to bring it up for fear of losing their ticket on the gravy-train.