OK, I will chime in.....
My expeience in this matter involves 45 Colt.... So, a few years ago I obtained some old 45 Colt ammo at the gun show. It was a mixed lot of really old stuff. It was all smokeless-powder, balloon-head cases, except for one. I fired most of it off, except for that one and a couple of others. The clue that one of them was a BP round is that it was the only one that didn't have a cannelure in the case where the base of the bullet would be. So, I dismantled a UMC , WRA-CO., smokless, and one WRA Co. black-powder rounds. Both brands of smokeless had hollow-base bullets and a small charge of powder that looked like it might have been Bullseye. The BP round had a 255 grain flat-based bullet over a compressed charge of 38 grains of black- powder. It was so tightly packed and compressed that the powder charge was a solid plug that could only be scraped out of the case wit a small screwdriver, and the base of the bullet was engraved by the pressure of the powder granules. The case was tarnished brown brass color, but clean and not corroded or pitted. The inside looked good, but not shiny. I chambered the empty case, and when I dropped the hammer it went bang!, with authority. I felt confident that the smokeless rounds were perhaps 80 to 100 years old, and there were several hangfires. I am certain the BP round was well over a century old and I am equally confident that it was functionally as good as the day it was made, muchmore so than the smokeless rounds it had been keeping company with for somany decades. I have absolutely no doubt, that well prepared, black-powder, ammunition, will outlast smokeless by a wide margin.