Another thing to remember is that Brompton actually looked into going over to Alu chassis (& hinges) but were not satisfied with the integrity of the welds each side of the hinge. These Mercs have not been out that long, how long, I wonder, before someone riding one sufferes a catastrophic failure?
As in all these things there are +s and -s to each and every folding bike, all have strengths and weaknesses not found in their competitors. All you can do is get the best compromise which suits your needs. Or, as I do, have different bikes for different functions.