A week ago overclocker der8auer made tech headlines around the world as he slapped Intel's X299 platform for its poor overclocking support. At the time he blamed motherboard makers for launching immature products with poor VRM designs. He also blamed Intel because the chip giant did not give motherboards makers enough time by launching the X299 platform two months earlier than expected.
After more extensive testing, der8auer uploaded a second video and overall he explains most of the initial issues he detected are still an issue. In particular, he notes there's a lot of VRM throttling going on due to the poor build issues on various X299 builds. However, he did back off his claim that the 8-pin PCIe connector on the board poses an overheating problem for moderate overclocking.
If you don't want to watch the video, TechPowerUp provides a nice summary over here.
His first major point is to watch power consumption. If it drops drastically, something is probably throttling. It does not mean (as some critics assumed) that the X299 system is not as energy hungry as reported: It is, it just has the nasty habit of not always reporting a throttle in clock rate. Furthermore, throttles can happen "back and forth" very quickly, and CPU-Z's refresh rate is not fast enough to always catch these. He recommends using HwInfo for frequency and VRM temp monitoring purposes.
Second, once you know there is a throttle occurring, there are two major types of throttling. The first is CPU throttling, which will occur via the multiplier backing off on the CPU cores slowly. The second is VRM throttling, which will manifest itself as a sharp drop in multiplier instantly (in his example, it went from 4.5 GHz to 1.2 GHz during a VRM throttle). These dramatic drops are related to VRMs and the poor build issues on various boards. Sadly, as of right now these issues still exist as a very complex minefield for potential buyers to navigate.