Both of the above statements are untrue except for APUs liking fast RAM.
2133 is faster. There is noting unstable about it and the lower the cas latency the faster the RAM to the order that DDR3 1600 cas 9 is roughly equivalent to DDR3 1866 cas 10 although it's not quite comparable that way due to bandwidth.
Any quality DDR3 1600 will be 1.5v and cas 9. Kingston is the only company for over a year now still selling DDR3 1600 that requires 1.65v or is cas 10 or 11 by default. This is either old stock or low quality or both. I avoid them.
With a traditional AMD CPU of the FX series DDR3 1866 is about as good as you can do because the memory controllers are on the CPU now and have been since the Phenom II for AMD and the first gen i5/i7...