I think the definition of 'pretty serious' trail varies greatly from person to person, but I can give you my experience with a fleet flatbed on a Tacoma over the past 4.5 years. I would not consider myself a 'four wheel driver' in the sense that the driving is the goal, but often to access remote places, you need to drive rough roads.
I have never found tipy-ness in terms of roll over to be a limiting factor, but I have found bulk coupled with off camber to be an issue. We have taken our last camper over Elephant Hill and into the Needles district of Canyonlands, but I would not do this with the FWC. Not so much because of Elephant Hill itself, but because of the 'squeeze' that comes after it, where you need to drive through a very narrow canyon, and it is off camber - the camper would likely hit the walls. I notice this with trees as well.
You also notice that you are heavy, it takes more traction to get you up a loose slope than without the camper - particularly with the Norweld, it is heavy! The weight also increases the likelihood of breaking something - it just takes more torque to get going and everything is working harder. So far we have been lucky, but it is something I think about, and I am much more likely to turn around with the FWC.
So in short, if I were seeking out four wheel driving for the sake of it I would look for something smaller and lighter. The FWC does add stress to really rough roads.