I have a crank on my two year old Thoroughbred filly.
And I hate it.
Why? Because I don't use it to crank her mouth shut, and therefore it has the same function as a plain flat cavesson, but takes twice as long to buckle up. I'm a very lazy person by nature so why bother? My gelding's bridles are both plain flat cavessons, and I can do them up and undo them with one hand if I feel like it, very quickly and very easily.
I can't do that with the crank.
MY reasons for disliking them are purely lazy, and also because they were designed specifically to crank the mouth shut and therefore the design ENCOURAGES over-tightening and misuse. Unfortunately it is very difficult to find a nice quality dressage bridle without a crank and a flash these days, and, stupidly, it costs MORE to buy a 'show' bridle with a plain flat cavesson [less leather by far!! also less hours involved with the manufacture] than it does to buy a dressage bridle with a flash and crank. Usually raised and padded, nearly always with piping and a blingy browband. No thanks. I prefer my plain flat leather. But why should I have to spend MORE to get less?