The stability recommendation mostly stems from the past, when people did not have all the modern equipment running their tanks. When i started in 2000 i did not have a dosing pump, digital thermostat or programmable lights. Let alone a refractometer or ICP testing. I was dosing by hand, had a simple thermostatic bimetal heater and lights was on a mechanical on/off timer, salt was checked with a plastic instant ocean floater at the start, later i got a hydrometer, which was not cheap back then.
We were just emerging from the time of T8 lighting and going towards T5, HQI was mostly not for beginners, but they were great at growing SPS.
Getting stability wrong nowadays is indeed becoming more unlikely, IF you can be patient and not screw with all your settings constantly. While at the same time keeping up with your standard maintenance on a regular basis.
I think the "idiom" is just as valid today as it always has been, its just how you interpret it i guess
I recently got a very nice example of how quickly a small change can make stuff go sideways.
When going on a 1 week trip abroad, my AC somehow produced an error and turned itself off.
Off course just in that week we had two very sunny 25+°C days, heating up the room to 27°C through the south facing window, which caused the tank to reach 27.5°C. Usually i run my tank at 24° in winter, and 25° in summer.
But i had not yet hooked up my cooling fan yet, since we had crap weather in belgium up until then.
Although there was only a 3.5°C rise in temperature for 2-3 days, i did see the results when i got home.
PC Rainbow, red and green dragon echinata, a pink mille and my SSC were all "bleached" or badly off colour.
FFWD 4 weeks and things are starting to get better again, seeing the colours get darker again day by day, but it will take at least 2 more months before i get back to where i was.
The few LPS i have, being a bernardpora, a alveopora, goniopora, and a hammer coral where all fine during this event. Had it lasted longer, i am sure they could have suffered as well for sure.