"if they have expired, they are not considered part of your legally required number of Visual Distress Signals."
That should be obvious.
Whether flares, old or new, are reliable and fire as expected depends on many things. Considering that they are not sold in airtight containers, and they are normally not KEPT in airtight containers, and even worse that they are kept in humid environments...
Whether your outdated flares are any more or less reliable than brand news ones, probably depends on how all of them were kept.
Sometime in the 1990s, IIRC, archaeologists recovered a Revolutionary War era cannonball from a mudbank somewhere in Virginia or the Chesapeake. The cannonball had been totally buried in the mud for 200+ years. When they cleaned it off, opened up the fuse hole, inserted a new fuse and lit it?
Yessir, that E.I.Dupont DeNemours And Company Cannonball exploded just fine after 200+ years. Nice layer of ud plugged the fuse hole, kept the powder dry, and that's all it takes to have reliable pyrotechnics. Just buy 'em from a good source and keep that powder dry.
And every three years buy three more toys to meet the USCG regulations. "No sir, you may not see them, they are hermertically sealed in this container with the rest of my pyros so they will work when I need them." The dayglo orange hermtically sealed can with an inventory on the outside seems to be sufficient.