If the luggage-truss (or whatever Brompton calls their system) was better than a fork-supported rack, then specialty loaded-touring bicycles would be using them. I use a fork-supported rack on my 20" bi-folder, instead of a luggage-truss, for several reasons. The rack has a much higher weight-capacity. The rack keeps the weight lower. The rack doesn't preclude the use of drop-handlebars.
As for rear-panniers working with a folding bicycle, some people have had success with this, some haven't. There are a couple of anecdotes of the former, posted in
another thread.