For me, the SA control ring on the 100mm macro is a pain. It has a lock on it, but once or twice I've inadvertently knocked it to the ON position when taking the lens from the bag. A few weeks ago I was working in the rainforests of West Papua, and got very frustrated one morning when all my photos looked soft.
I checked and cleaned the filter, front and rear elements, and even the sensor, as I'd wrongly believed that the softness was caused by condensation (a common problem in rainforests, as you can imagine). Next I swapped lenses and fitted my RF100-400mm, and magically I suddenly had bright, crisp, sharp photos. It then suddenly dawned on me to check the SA control ring on the macro, and indeed it had shifted, and was the cause of the fuzziness.
I believe that Canon fitted the SA ring for the benefit of portrait and wedding photographers, who may favour the "soft radiant glow" look, but that's NOT what I want in macro work, and the lens is specifically stated to be for macro. There are plenty of other lenses more suitable for wedding/portrait work, so I find it frustrating that Canon poked a SA ring on the macro.
I find it even more frustrating, that while they put an unwanted "extra" on the macro, they completely failed to put a function button on the RF100-500mm (although just about every other recent long zoom on the market has one).