There is one other big advantage held by the S12II/M12II: Ripple suppression. At 500 w and under, the S12II/M12II have crazy low ripple that often doesn't even break 15mw. The CX can't touch those numbers. In fact, there are a few $150 PSU's that can't touch the S12II and M12II ripple numbers. It beats the CX as hard in ripple as the CX does in voltage control.There's one thing about the S12-II/M12-II that's not yet applicable to the new CX/CXM. Time in grade. Those old Seasonic GB Bronze platforms are solid, been plugging away for 10 years. Not uncommon for them to last double or better their warranty. They may not have the performance of the new CX series (or some of its protections) but they are workhorse psus none the less. Since the CX has only been around since 2015/2017 time will tell if they'll last the same, when put to the same levels of abuse the Seasonics have been dealing with since 2009?
Another thing I always liked about it is that you always know you are getting with the S12II. Buy an S12II in 2007 and 2019 and you are getting more or less the same great quality. You could fall out of the loop for a few years and still purchase the S12II or M12II with confidence. The CX has changed so many times over the years that the only thing you knew for sure without updating your research was it wasn't total crap.