I think a factor is the weight limits of bikes and the fact this information isn't clearly presented to the customer. Lower weight bikes are more pleasant to ride but often lead to lower weight limits and a higher risk of failure. Generally Dahon, Tern and Decathlon Btwin bikes have low weight limits and also suffer from the most recalls for frame failures.

In the UK we have a large chain of shops called Halfords that sell their own brands of bikes, they state a maximum rider weight of 120kg with 20kg additional weight loading away from the seat tube for adult bikes. They also state an absolute maximum 160kg loading that you must not exceed and rarely if any suffer from frame recalls despite having a large percentage of sales in the uk for bikes. Decathlon state 100kg maximum load weight but also include the weight of the bike meaning a maximum rider weight including clothes, luggage etc of 80-90kg depending on bike type. You'd not know there was such a difference going into either store as this information is concealed in the manuals and nowhere else.

If it was the law that every bike had to have a clear maximum load label on its frame we may find many of these recalls would disappear but also sales of low strength bikes would significantly drop.

Dahon is a confusing brand because I think their bikes have been all over the place regarding strength. Some of their non peformance early steel bikes seem strong, their more recent performance bikes are fairly weak, the new Curl looks strong but I seem to remember they did an earlier chromoly steel peformance bike that was actually quite weak with thin tubes. I don't think Tern has ever made a strong bike, they are clearly focused on the performance side of bikes.

Clearly many of these comments are about the durability of hinges of folding bikes. I must admit I'm unsure reading the comments if the failure of these is purely through the wear or fatigue of purely opening and closing such hinges or the fatigue of the hinges flexing while riding or a combination of the two. I'm sure though the weight of the rider in many cases is a factor. In the old days you used to get over-engineering allowing for the worst case scenario and making something strong enough to cope with that. Many of the early steel folding bikes of 50 years ago are still going strong and likely to outlast their modern aluminium counterparts despite the age difference.