Look at some serious off road activities like Moab, Rubicon trail, etc. Those Jeeps and FJs all have spare gas cans strapped to their racks for one purpose only...in case they need to be recharged. IMHO, being, "really capable off-road," means sustaining a couple days on the trails with what you got. Will this sustain its charge long enough to do that? It would be bad enough to lose your charge sitting in a traffic jam on a cold day in the city. I can't imagine losing it in a serious trail ride miles from civilization.
I suspect “off road” for most of these vehicles will be mall parking lots.
I agree, as someone who has Rotopax mounts on both my vehicles and has had them save my bacon. I can see someone out in the bush with a trickle solar charger camping for a week to have enough to drive out.
This might be good for occasional trail users who don’t cruise far from home. Most of the guys I wheel with are within an hour drive of their house and don’t carry extra gas.
The other problem is it’s weight. A lot of the time power does not help you if you’re too heavy. The ‘most capable’ offroaders are usually light. Depends on what sort of wheeling you do though.