Like a lot of things, "it depends" - on a lot of factors. I have a 16" shank, and it works fine for my vehicle combination (a 23' Safari and a GMC 2500 diesel) - the truck has the suspension to handle it o.k., and I never have to worry about having a corner of the trailer and a corner of the truck meet when backing, even in full-lock turns. And I can open the truck's tailgate without it hiitting anything on the trailer.
BUT - the longer the shank, the more load on the truck's receiver, and the more opportunity for sway. Also, since it "sticks way out back there" it is more exposed when there's no trailer attached back there, and is easier to run into, both for vehicles and pedestrians. Never had it happen, but I'm just sayin'
So - it's surely not for everyone, but it works fine for me with my relatively small trailer and relatively stout tow vehicle.