Aluminum is used on heavy duty bikes, e.g., most MTBs. It is in fact more stable than ChroMo per weight, thats why.
The Dahon hinge has been absolutely trouble free for me. After 2 years and two bikes (one got stolen) not a single sign of fatigue. The new hinge design might be slightly improved, but anyway the older one is just fine too.
My Mu Sl has been on par with full size bikes, performance wise. I did take part in group rides and did lots of races with messengers on it. I did not see speed disadvantages of it beeing a folder. In the city, instead, it seems to have the edge and is increadibly fast, one of the fastest bikes in town. I doubt, a Brompton can be ridden to that level. However, I like its small fold.