I'm only seeing around 1/10th the expected amount of RR suppression from the "listed" values for the units. A 3XGAR unit should reduce RR by 6 (3 units X 2 reduction per unit), but I'm seeing more like 0.6 in the province display.
At this point, other than using GAR units as a cheap way to guard locations, even MPs don't seem to have enough effect on RR to make them worth building for suppression purposes.
Rather than the "fixed" 0.3 or so per year drop in Nationalism, it would be a really nice "feature" to have that and RR "absorbed" by suppression units, sort of like supplies in reverse. Over time, you could increase the drop rate in defended and surrounding provinces with heavier garrison and MP placement, causing a gradual decline in suppression of provinces still further away over even more time. After a few years, you'd have "safe" control of the cities and surrounding zones, with higher risk in the backwater areas. Leaving a wide area unsuppressed, or only marginally occupied, would mean an ongoing high risk in the entire region throughout the game.