FYI, I've stumbled across what may be a bug in the code that saves changes to the Field Groups.
I created a FG with two images and two rules, both with Page Template equal to a unique template. Match was set to any of the above.
This worked fine until I decided to swap the order of the two image fields. Drag'n Dropped the second above the first, hit Update to save the changes, and the FGs disappeared from all the pages which used the two templates.
Fiddled with reordering the fields to no effect. Then, thinking the match code was preventing the from showing up on the pages, I deleted both rules and then re-entered them, and the FGs started appearing on the pages again.
Have not tried to repeat this (too busy at the moment to mess with it), but didn't want to forget to report it (actually, I already forgot for two days, but better late than never, right? :) ).