There's a lot of truth to it, mixing set can be problematic, even of the same exact model, and the likelyhood of having problems increases as the freq goes up, that's why DRAM is offered in such a wide variety of number of sticks per package, those sets each have sticks that are tested to work together, and why prices are higher for the larger number of stick sets....Also of note with you, and 2400 sticks, XMP is programmed by the package so it may well cause problems if you try it, i.e. a 2x8 set of DRAM may use a tRFC (one of the many advanced timings) of 220 while the same sticks in a 4 stick config will require a tRFC of 314, other advanced/secondary timings may vary also (though none as extreme as the tRFC). Another thing you can run into, particularly with higher freq sticks is it often takes voltage adjustments to DRAM and MC (memory controller) to get them to play nice, with the higher freq already at 1.65, don't have a lot of wiggle room