Due to their popularity I suspect most people will say I'm being unreasonable to dislike extendable dog leads but here are my issues:
When they're used near roads, unless the user has them short the dog can easily go into the road. It's as though owners think they have control because their dog's on the lead but in reality... I'm extra cautious driving past a dog on a flexi lead for this reason.
Similarly, when used long on pavements the dog can go around a corner way ahead of its owner and straight into a pedestrian or another dog (I've been in the receiving end of inattentive owners with their dogs on long flexi leads a few times!)
They easily get wrapped around people and other dogs and are difficult to use safely. The thin cord can cause nasty injuries
Using a long line for teaching recall, or allowing a dog with poor recall some freedom, is better IMHO.
The constant tension on the lead isn't ideal from a dog training view point apparently (which I can understand)
What do you reckon?