If the dog runs full on to the end of the lead and doesn't stop then the lead can snap
I didn't think they were designed to be used at maximum extent - the idea is that they move in and out loosely with your dog's movements, and you call the dog in the normal way to bring him back (but have the safeguard of being able to lock the lead immediately if he doesn't respond and is straying where he shouldn't).
I understand that there are plenty of irresponsible owners who'll misuse this type of lead, but are the consequences any more dangerous than those of irresponsible owners letting an inadequately trained dog off-leash altogether?
If we accept that, like it or not, there are irresponsible owners, extendable leads would at least seem to mitigate some risks -
If an untrained dog, off leash, runs towards the road 10 times, that's 10 times he will end up in the road.
If an untrained dog on an extendable leash runs towards the road 10 times, 9 times out of 10 the lead will stop him, on the 10th time it might snap - that's still 9 times an accident has been averted.
I agree there may be different risks - eg. the trip risk with extendables mentioned by a PP, but I do think this is an owner-behaviour issue, like so many dog-related controversies, and not an inherent fault in extendable lead use.