The majority of the dedicated trail systems require a ATV or OHV pass and in order to get a pass or permit there are restrictions regarding dimensions including engine size.
Quite a few states and some provinces have a 1000 cc limit and that is the reason that you see the manufactures are now in a horsepower was using turbos.
I know that in Ontario you can green plate (OHV) a tube buggy but not a vehicle that has 17 numbers in the VIN and is or has been registered as a motor vehicle.
In New Hampshire, Quebec and Ontario I have had to surrender the registration in order to purchase the pass and they confirmed that the vehicle was registered as a OHV and had valid insurance.
As far as crown land trails here in Ontario that do not require a pass and are not part of a club system then you are correct you could take a Sami or Jeep etc on the trails but not where there is a land use agreement.
As I said in my previous posts I am a Jeeper at heart and always have been but these side by sides are a whole new experience. In order to get the kind of power to weight ratio and suspension travel out of a Jeep I know you would have to spend a whole lot of money but then again windows and air can be nice to have from time to time. LOL