I'm not a hockey coach, I'm a football and basketball coach so I'm applying some logic here that might not completely jive but....
I've heard Maurice talk a lot about the man-to-man setup being easier and that he needs to see the team show an ability to grasp the concept and execute it before moving to something else/they devolve into it when things are not going well.
I understand logically that man-to-man is simpler, you take this guy, you take that guy and so on. But in terms of execution there is no doubt that man-to-man is far more vulnerable to lapses as there is very little safety net. One guy gets beat and another guy has to decide to keep his guy or move to the other guy and it just cascades from there. Usually zone defense concepts have more of a flow for dealing with that and regrouping. Can't say our man-to-man looks like it does.
Him saying it's simpler and he needing to see more is just a bunch of horse crap. These players are all highly trained professional athletes who would have played pretty much any variation of defensive systems from junior/college to the NHL.
If he feels they can't do it tells me more about his inability as a coach then it does about any of the players. All of these players are capable of easily learning any NHL system if given the proper time direction.
You are right that M2M may seem simpler in theory but it isn't in practice due to how quick hockey is. I'd imagine the other two systems that are predominantly used would be much either to execute once learned.