Motherboard and Memory Compatibility

Well, I guess this could be more focused on memory, but it does deal with motherboards aswell, and there is no "memory" section. Anyways, my question is.. Is it "risky" to install for example 2133MHz RAM into a slower motherboard memory slot whose standard is either 1866, 1600, or 1333? Also, when I say "risky," I don't mean will it like blow a motherboard up or anything, I'm asking if there is a pretty much guaranteed chance it will work on just about any board with those standard RAM speeds, just defaulted down to the standard? Or will have to settle with an ASUS board with MemOK just in case? 

from what i've read, it will run at the highest that the board can run. so it would be limited to 1866. But i would get a second opinion on this.

Well most of the board I see can overclock to 2133 or so, and I have heard of people using 2133 RAM in a 1600 standard mobo. I'm just wondering if it's difficult to get working on most boards.