In some, but not all hooked breech guns there is an internal configuration that slightly slows the process of the powder burning. For instance a TC hooked breech percussion gun. The flash channel is about a 3/16 hole drilled from under the nipple to the center of the breech plug. The breech plug is hollow and has threads that extend into the barrel. This hollow part with in the threads is about an inch deep and about 3/8 inch in diameter. (The breech plug is actually about 1.75 inches long. You don't see the threads that extend into the barrel.) As a result you fire the gun, the fire must burn through that flash channel which is only 3/16 inch diamter, unil it reaches the 3/8 inch diameter hollow in the breech plug that is nearly an inch long and then reaches the main charge in the 50/54 caliber barrel. Now that burning through the flash channel and the narrow hollow inside the breech plug, probably adds a miniscule fraction of a second to the time between hammer fall and the ball exiting the muzzle. If you are the type of shooter that moves as he shoots or flinches, that is a small fraction of a second to pull the shot even further than it would be in a flat faced breech plug type barrel. Now if you have perfect follow through after a shot, there will be no difference down range. However, if you move slightly, there will be a slight difference on the target.
So the gun itself is not more or less accurate, it is the very slight difference in ignition time in the hands of a shooter that moves, that makes the difference.
There have been some hooked breech systems that have flat faced interior breech systems. My Brother has some crazy foreign made muzzleloader in which the breech plug is simply a short threaded piece with a hook, no octagon shape. The barrel extends all the way back to the base of the hook.
This mechanical/design difference isn't present in some rifles. For instance an underhammer, and fans of the underhammer claim they have faster ignition than percussion guns with hooked breech or drum and nipple. The fire goes immediately and directly into the main charge.