Articap said it well, "a poorly designed patent breech may be worse than a well designed drum and nipple". I would add that the most common patent breeches, such as T/C and its Italian cousins are poorly designed. They move the powder charge farther from the ignition source and provide a very small passage between nipple and powder chamber. That flash channel is only about 1/8" diameter. Powder will not, cannot flow into that tiny passage, so the fire from the cap must turn 90 degrees at the bottom of the nipple and jump through that tiny flash channel to reach the powder. Add a bit of fouling coating the inside of that 1/8" hole and you have virtually no passage. That is why you'll often hear those rifles giving slight hang fires, just enough hang that you can hear the pop of the cap distinctly from the boom of the powder charge and why those rifles often have trouble with substitute powders such as Pyrodex. If the flash channel could be enlarged to 5/32 or 3/16" it would be great but in those common designs by T/C or Investarms it cannot be enlarged. A nipple drum may also have a tiny passage, but one having 5/16x24 threads or larger can have a passage large enough for 2f powder or Pyrodex to flow right in under the nipple for fast and certain ignition, just like an underhammer. But the drum must be properly designed and fitted up.
For those interested in blowing up rifles a patent breech is stronger than a drum & nipple but the properly fitted drum is more than strong enough for any reasonable charge in a round ball rifle. Long range rifles firing heavy powder charges under long, heavy bullets produce very high pressures and are safer with a patent breech, plus, that is the way those late period rifles were traditionally built.
