I can track-stand my Brompton nearly as well as my mountainbike. And they do get more stable at speed. Also, strangely, they get more stable when you add load to the front carrier. But they definitely feel more flighty than most bikes. I wouldn't dare ride mine no-handed. They require at least a light touch holding the bars all the time to keep them going in the intended direction. You need to provide very little input to produce quite drastic direction changes. They definitely aren't as stable as many bike designs.I wonder if any of those saying that Bromptons are unstable have actually been for a proper bike ride on one.