Its not a matter of whats different, but that it is. The certification process used to be just a matter of documenting properly modifications or common systems as they exist. Having a customs atorney for the first few was always necessary.
We brought in about 30 cars in a couple years starting in 1984. The venture stopped when the feds buckled to manufacturer claims of discrimination. The manufacturers whined that their systems had to last 50K for some huge portion of the fleet. They couldn't understand how the government was letting hucksters whip a car through testing that didn't have a chance of making it around the block.
Some where along in the late 80's the deal was changed and any converter had to act as a manufacturer with destructive testing and model certification. The process was left to a few who wished to do just a few models.
Bosch Master, ASE Master, L1
33 years MB technician