Flexi leads are not great for a few reasons. Firstly, in order to extend the lead, the dog has to pull against the lead. That teaches them that pulling is ok, then we expect them not to pull when a normal lead is used. Secondly, they tend to be very thin lines and can get caught up in things, or wrapped around things (including people) very quickly. They can act as a trip wire. Thirdly, often the handle isn't built for the job, and although it can cope with the pulling, it doesn't always cope with pulling and twisting, which is what happens when a dog is straining against the lead. People can find themselves holding half a handle and not much else. Finally, if your dog gets itself in trouble, then trying to get them back with a flexi lead can be difficult.
A long line is very different. You do need good mechanics (you need to let the line out as the dog walks away, and gather it up as they return), but in an emergency, you can just step on it to halt the dog, then gather the line as you walk to the dog (never pull the dog back to you).