I learned this the hard way when I bought a 2x16GB registered DDR3 kit, and then finding out my i5 won't boot with it. It was next to impossible to find unbuffered memory with 16GB per module last time I checked.
They do support unbuffered ECC, which is pretty great, but at least the v2 units I've got are limited to 32gb ram, which stinks.
It's the type of ECC RAM you need for E5 & E7 Xeons - multiple physical CPUs. Many of these systems won't even boot with normal DDR3 sticks, ECC or otherwise. Must have registered DDR3 (and I suppose DDR4 now).