I regularly shoot caps I obtained in the late '90s, acquired from the estate sale of a friend who bought them in the early '80s. I don't recall any misfires due to the caps (I've got a pepperbox that is not 100% with anything, still working on sorting that out). Some of my caps stash date back to the '70s with no problems.
Around 2007 I set up a test to compare then-modern caps with original Eley caps in original and repro revolvers. I tested Colt's 1851 Navy and 1860 Army, and repro (Uberti) 1851 Navy and 1860 Army. The Eley caps came from sealed tins marked "250 metal lined caps made exclusively for Colt's PT Belt and Pocket Pistols". Modern caps tested were from CCI, RWS, DGW, and Remington.
Each gun was fired 125 shots with caps from each maker. Guns were fired from a Ransom Rest; each round was chronographed. No Eley caps failed to fire; the others all had a few misfires. Velocity variations were rather smaller than I expected, but consistency in velocity, the modern DGW caps were #1, Eley was #2, and RWS was #3. If I needed caps, I wouldn't hesitate to use any of them (keeping in mind that the Eley caps, as well as other early caps, were corrosive).