When Skylake was released, Intel announced the move to the new DDR4 standard while maintaining limited support for DDR3. Initially, evidence indicated that Skylake could support only DDR3L, but since that time, motherboard OEMs have listed support for non-DDR3L RAM, which raised obvious questions. After speaking with Intel about the issue, we finally know the truth about what RAM Intel's integrated memory controller (IMC) can support.
Skylake's IMC officially supports only DDR3L with a voltage of 1.35, and it officially supports DDR4 at 1.2 V. This might seem a little counter-intuitive to some users, as the entire reason for continuing DDR3 support is to make it cheaper and easier for customers to upgrade to the new Skylake platform, but these voltages are likely used to avoid damaging the IMC. Although it would be nice to use any DDR3 from 1.35 V - 1.65 V, running at these higher voltages could burn out the IMC and kill the processor over time.
If this is the case, though, then why do OEMs such as Gigabyte support DDR3 at 1.5 V on some of their motherboards? And why do others such as Asus and ASRock support DDR3 at 1.65 V? RAM running at these voltages might be capable of operating on the motherboard without causing damage to the board itself, but again, over time it will likely damage the CPU. So if you don't have DDR3L on hand, you are probably better off to go ahead and spring for the more expensive DDR4.
Another interesting point that we learned from Intel about the IMC is that it only supports clock speeds up to 4,133 MHz. Although DDR4 is still relatively new, and most RAM kits currently run between 2,133 and 3,000 MHz, some RAM kits are already reaching beyond 4,233 MHz.
RAM kits running at clock speeds above 4,133 MHz will likely encounter more errors than other DDR4, and it is probably best to limit yourself to memory running below this clock speed for the time being. DDR4 at higher clock speeds will become more useful in the future as Intel refines its IMC to support faster RAM, but that isn't likely to happen until a Skylake refresh or the release of Cannonlake.
Long story short, if you are looking to buy a Skylake platform, make sure you double check the RAM you are buying to avoid having problems down the line. Even if it works initially on your system, there is no guarantee that under stress the IMC won't break down and knock your system out of commission.