I don't find it difficult to believe that none of Negan's men have tried to kill him because....why would they? Why would they rock the boat that is giving them everything they want? Weapons, food, sex, booze....on and on. They take whatever the F they want when they want. Negan has established himself to them as a leader that provides. I don't get why we'd assume that one of them would want to take him out?
Gary, my bro, my buddy. I think I can help you out with why some (most) think the Negan as a leader situation is unconvincing, but I need you to put on your imagination beanie.
Imagine you, Gary, have been swallowed up in the Saviors. Your wife is being held as a harem slave by Negan. Your boys are all dead having been carried away by walkers years ago. You've been through unimaginable pain, years of the roller-coaster of depression, violence, hope and anger yet here you are; a survivor. You've seen Negan smash heads up close, he's personally told you your wife's life depends on your good behavior.
One day there is a call to arms, the whole clan is to terrorize a new threat and bring them over to the Saviors. You're standing there watching him do his schtick. You've got an automatic weapon in your hands. You're standing 12 yards away. The safety is off...
The problem isn't that there are people who want the perks and are willing to go along. I get that, I really do. The problem is that in a group that size, considering Negan's "management style", that we're expected to believe there isn't one person who would just say (or hadn't already said) fuck it. Even the Governor was smart enough to give the illusion of peace and prosperity.