As to the aftermarket parts for Brompton, in my personal experience the vast majority of them does not work as they are supposed to, maybe 9 out of 10. When you inspect them you see that their designers do not even understand the functioning of the bicycle. My suspicion regarding the bikes put together out of such parts is that their owners hardly ever use them and their primary purpose is to show them off to the friends putting together similar unusable bikes. The aftermarket of folding bike parts is an out of control horror, there to generate funds for the manufacturers and resellers but with not much respect for customers. Anyway, that's my personal experience so far.
Nope, most of the people I know riding modded Brompton have quite a bit of mileage (both horizontal and vertical). Many do randonneuring with their Bromptons. One of them even completed Paris-Brest-Paris last year.