Personally IMO, for all the types of off roading most would do the ubolt method would suffice.
Subframe mounting has been covered on this forum many times and it is often a very contentious subject.
The bottom line is that there is no "one size fits all" solution for mounting a subframe/camper to an expedition vehicle, as there are too many variables: brand and year of truck, chassis design and thickness, weight of the camper, terrain that will be traversed and the list goes on.
You are correct that a fixed mount could possibly do the job, but in an off road scenario the chassis will twist much more than a road going truck would experience. If that twisting is only encountered occasionally then there would be less risk of chassis damage, but it's like asking... how long is a piece of string?
On this forum there are numerous pictures and videos of Fuso/Isuzu owners doing some decent off road driving and what is evident is some pretty serious flexing when they do that.
Especially with the stepped frame of the Fuso, a fixed mounting system will create some pretty serious stress points, normally focussed just in front of the step. I have seen many trucks with cracks in the chassis in that area.
Personally, I would not like to go off road in a Fuso that had a fixed mounting system, but maybe that's just me. Each to their own...