Well, I think that's easy to explain. Those two simply represent the more extreme and potentially more degrading side of the spectrum.
From what I've seen on this forum so far, the most common complaint about Nathan from the cuck side is that he's too vanilla. This is not an unfair description, but at the end of the day, while he's too tame for those people, at least he's another bull for the FMC. On the other hand, the Stags I've seen complaining about Oliver (and, as you said, to a lesser extent, Mike) are reacting more or less along the lines of "I never asked for this!". This isn't surprising to me because I'm pretty sure the Stag group includes many people who are new to the fetish or prefer the "lighter" side. Those who are not just playing the Stag path to be the dom, but also, or maybe solely, trying to control that they don't bite off more than they can chew.
So, if one side's main complaint is that things aren't going far enough for their taste and the other side's complaint is that things are going too far, I'm not surprised that the first group is mostly just reacting with mild annoyance while the second group is sometimes reacting much more passionately. The first group is still getting part of what they want, after all, while the second group is in over their heads.