In all the years I've used an extending lead with my dog I've never had one snap, nor the locking device fail, but then I don't use a metal one, which do tend to snap.
My dog doesn't pull on the lead whether she's on a normal fixed lead or on the extending lead, it makes no difference because she's been trained not to pull.
We have lots of fields with high banks and deep water courses, in our rural surrounding where I sometimes walk her so in those places I need to keep her on a lead. If she wasn't on an extending lead in these places the walk would be very boring for her and she wouldn't get to explore.
I find in certain circumstances an extending lead is a great tool.
The trouble is that some people don't use them correctly, which is when problems occur.
We use a lot of things in our lives that used incorrectly can cause injury or death: e.g. knives, cars, hedge trimmers, irons and so on, we don't tend to claim they're a terrible idea because some people don't know how to use them in a correct and safe way.
It's the same with extending leads, some people won't like them, but that doesn't mean they're generally bad or dangerous. They just need to be used safely and properly and in the right environment, which is probably not a busy city street.