"But failures will happen as the systems cannot provide perfect operation in all conditions. And the vaunted COSPAS-SARSAT PLB is no different - you just don't see it tested like you do InReach and SPOT."
Multiple satellite constellations. Multiple satellites in each constellation. Two GEO satellites covering North America, with a spare in orbit on standby.
Multiple coordinated LEO ground terminals. Multiple coordinated GEO ground termials. Multiple Mission Control Centers. All in sync, and handling the vast majority of validations and communications via automated systems.
Testing? There are pages of PDFs out there with the results of testing - like 25 concurrent signals in high interference regions, etc. etc. This is government (including military), and you are suggesting that testing isn't thorough?
They aren't driven by ROI, P&L, or subscription efficiency. They have published operational standards, they test for them, and report them. No one bothers to look, or even care, simply because things work.
I'll grant there Can be failures, but that number is disappearingly small.