Their usefulness depends largely on the intended use. If you are expecting exceptional accuracy with a pistol caliber shotgun shell sized insert out of a shotgun barrel,..it isn't there,..nor was it ever intended to be,..though with the longer barrel inserts it could be.
The shorter inserts used in shotguns are able to be used for taking very close shots at small game,...maybe,...but more importantly, the are meant to be used to "dispatch" larger game with the ammo from a smaller caliber handgun.
The longer rifle and even pistol caliber inserts in break down shotguns are in fact capable of fairly reasonable accuracy close in,...not MOA for sure,..but,..minute of deer at 50 - 75 yards,....very doable.
Some folks find rifle chamber inserts to be very accurate when used for smaller game rounds.
They do work,..but ,...think about that intended use,..and get the one that is intended to do THAT job,......they were never intended for rapid reloading or continuous firing.
If you have never tried one,..you owe it to yourself to do so, before you pooh pooh the concept. YMMV