Hooked breeches have the 'potential' for being less accurate than a fixed breech. The more moving parts, the bigger the chance of the barrel moving shot to shot and therefore changing the recoil patterns of the barrel/stock fit. Properly breeched, as many do, there isn't a bit of trouble with them. I doubt anyone would see the difference if a gun was made and tested both ways, first fixed, then hooked later after working up it's best loads.
How many spend the time to work up a rifle's best loads in the first place, rather than simply merely picking a load and using it?
My 14 bore rifle still shoots into 1 1/2" or smaller at 100 meters and it's hooked, built by Taylor 23 years ago. I've been unable to shoot that well with the fixed breeched guns I have - does that mean the hooked breech is more accurate than fixed? No - just that so far, that rifle is more accurate, or I am able to shoot it more accurately or that I've spent longer finding it's accuracy loads.