I think a lot was down to the build. Just a couple of years ago, I found a Moulton on the kerb waiting for the scrap men. I grabbed it and got it back into use. My mum still rides it. The rear wheel was rusted beyond help so I had a go at building a new wheel. I ended up sourcing spokes for the 16" rims from an old chap in Bilston. We got talking and he reckoned he built the wheels for my bike. He did all the high end Saracen stuff back then. I must say he did a very good job.
OK, I weighed a lot less back then but, those wheels took some use/abuse. Racing, riding down steps, touring and endless riding. They stayed pretty true and even if they did go out of true, they were easy to get back.
I really liked them.