I think CRM can be a serious issue in GA flying - if you have two qualified pilots.
Not so much on aircraft control when airborne - the two are hardly likely to be fighting over the controls - but in the go/no-go decision before the flight, and in exercising various options w.r.t. weather ahead when airborne.
I've done some stupid things when flying with another qualified pilot - in these cases it was an instructor apparently much better qualified than I was (I am not and have never been an instructor; not even a CRI).
I remember flying into a SE UK airport once, and flying the last half an hour in IMC, with the ILS flown through the bottom bit of what turned out to be a huge very dark CB. Pretty damn difficult. Then, just to make sure I didn't go to sleep, a circling approach to land (yeah I really love doing a RH circling approach especially in these character-forming conditions). The wind at 1000ft was about 60kt across the runway, within limits on the surface. I let him land the plane.
If I had been alone, I would have taken exactly one look at the MSLP chart and popped down a local bar instead (it was in a pretty pleasant European city).
A really stupid abdication of responsibility, thinking that the other bloke knew something about weather that I didn't know.
And I've done this kind of thing more than once. Get an instructor in the plane, even as an informal passenger, and I end up taxiing down the wrong taxiway, almost guaranteed
I think a lot of pilots chuck away a lot of what would be their normal planning decision rules when they fly with somebody who they think knows what he's doing. Might even happen with two plain PPLs because each may think that the other one would "say something" but neither says anything to the other.
The other bit of "CRM" which is really crucial is to keep decisionmaking
in the cockpit, meaning do not let ATC push you into a corner. It's happened too many times. If there is some serious weather 20nm ahead and ATC will not grant a 30 left/right etc despite repeated requests (never had this happen in the UK) then declare a pan or a mayday and just get on with it. Enroute ATC have radar and they are paid to sort it out. And if refused a landing (e.g. due to lack of PPR) and you are on the minimum fuel to alternate, declare a mayday at the slightest delay and land. Work with ATC but do not be afraid to make the key decisions.