It was my understanding that if PCM has bad data it will supply the servo with the last good data until it goes in failsafe.
Yes, that is the way modern PCM Rx's operate. To use a typical "glitch" counter with a PCM Rx, you merely plug it into an unused channel. Then, program the channel to revert to the opposite direction upon failsafe. Using this trick, the Glitch counter operates as a "failsafe counter." That is as useful as it is expected to get.
I have never used Jomar's Snitch, so if it somehow can detect realtime glitches from the PCM Rx then I would certainly like to know what/how it does such a trick. I never say never ...