You can't compare a Taiwan made Dahon with a British hand-crafted bike like a Brompton. I don't know where you get the idea that Bromptons flex horribly? My Lightweight Titanium is perfect and rides great. It is only a 2-speed but that is what I custom ordered. I wanted a fast, light bike for commuting by train. I also have a 2013 standard steel S-Type, 6 speed Brompton which I bought for use while my normal railway station was being refurbished and meant I had a steep hill to climb each morning... used it for a few months then went back to the lighter titanium Brompton.
I've test ridden plenty of Dahons, Birdy's and a few other makes and I can confirm that the Brompton is by far the best
Fast, compact fold ideal for storing between the back-to-back seats of trains. I even beat "roadies" on their racing bikes sometimes just for fun
I'm a Apple Mac user and there is a saying "once you go Mac, you'll never go back".... and its similar with Bromptons.... they are like the Apple of the folding bike world.... anything other than a Brompton is just a folding bike
I got the idea that they flex horribly after riding one in a test ride. The Mu is definitely stiffer and more ergonomic.
Bromptons definitely fold better than a Dahons, but they ride way worse.
BTW, I don't have anything against UK or Taiwan, and after a few years working on a Mac, I learned to hate them. They're just overpriced hardware with crappy software bought by people who are unable to to get near a PC without it getting full of browser toolbars and malware, which I couldn't do it even if I tried. So yes, I went back and I'm not regretting it.