Well, I was gifted an x299 motherboard, MSI SLI plus. So did a quick system build with a i9-7940X and a H100i-v2 cooler. Was happy with the CPU results, under high load, jumps to about 52c, then drops right back down when load is reduced. So happy enough on that end. Until I felt around on the mobo at the VRM heat sink. 
 
Holy smokes, it hits over 63c, per the mobo's onboard sensor just sitting there idle. And the really bad part of it is they went through the trouble of making a honking big extrusion for the top side of the board, replete with RGB lighting badges to further inhibit airflow. However - the back side of the PCB contains the other half of the VRM mosfets and they are cooled by.... a flat metal plate. Literally, 100x15mm flat metal plate, no fins, nada. Just sandwiches to the aforementioned cosmetic sink on the other side with two screws. One side gets cool-looking big metal thing with lights, other side, with just as much cooling needed, gets a 2mm aluminum flat plate. 
 
I have to say, this is one of the dumbest things I've seen. 
 
It works, and the mosfet sensor reports that it never goes above 75c (so far). But that's just nuts. 
 
Otherwise, boards seems fine in most ways, just getting used to MSI's bios. 
 
Will be thinking over whether to keep it or move on to something else. 

MOSFETs are designed to sink their heat into the PCB. With proper PCB design, large ground planes, adequate VIAs, and properly spaced thermal load, large heatsinks on MOSFETs are unnecessary. MOSFET heatsinks are inefficient in the first place. MOSFETs don't efficiently transfer heat through their insulating tops, plus the thermal pad or paste adds additional thermal resistance. Often MOSFET heatsinks serve more to just even MOSFET temperature between phases for proper MOSFET phase load sharing. Without even MOSFET temperatures, the hotter MOSFET would have decreased amperage output and the phase outputs would be uneven. MOSFETs are often rated for more than 110c, but it is good to keep their temperature below 105c in order to keep nearby capacitors soldered to the same PCB happy. If you are seeing MOSFET temperatures below 100c under load, there is no need to worry. I think products are generally engineered effectively, and generally there is no need to question why they are manufactured a certain way; a product with a warranty would lose the company money if designed inadequately. Game/crunch/produce/whatever without worry.

No I understand how the mosfets are rated and how the heat tabs are soldered to the pcb.
 
On the other hand - they didn't use the best FETs and the heatsinks are dissipating a lot of heat even if coupled to the epoxy side of the packages with a gel pad.  Honestly, something that's not really pulling that many watts at CPU idle, shouldn't be hovering at 65c in a cool ambient environment.
 
The goofy part is half the fets are under the rather large heatsink you see on top with RGB lighting and everything. The other half are on the back side of the board, under a thin 100mmx15mm metal plate.  (on the left) on the image below. Yes, it gets really freaking hot.

Who needs cooling when you have RGB?!
 
I stay away from anything with RGB.
My X99 Classified runs perfectly and I won't be upgrading until EVGA releases another "Classified" board that I like.

The RGB thingie actually blocks the airflow through the heatsink. I took it out - and dropped 7c at idle...
 
Yes, I know about VRM's and that mosfets can handle substantial heat. The onboard sensor says it is 63c, idle, doing nothing. That's a little wacky. 
 
There are competing boards (even from MSI), which don't double-side the VRM and have more effective heat dissipation. Not all fet modules dissipate heat through the bottom solder tab either, though these do. There are also better fets than those used (they used PK616 and PK632, 16 on top and 16 on bottom for pete's sake) that could all be placed topside and in contact with cooling. A better FET choice would have all fit on top and run substantially lower temperatures. 
 
Also, over time, even though the fets themselves handle 100c or more, the board around them, including the solder joints and the substrate itself suffers. 
 
They (MSI) offer this as a workstation (design station) board, not just a gamer board intended for intermittent use. Long hours, high CPU loads. 
 
I think it fair to criticize it, speaking as an EE myself. 

Point a case fan in the direction of the heatsink

So I am going to replace this board pretty soon. 
 
In further testing, under moderate compute loads, the VRM sensor hit 98c in about 10 minutes, with the LED strip taken off of it. It was continuing to climb. That's with a i9 7940x running at 3.1ghz - no overclock at all.  With a 40mm fan strapped to the heat sink, the sensor reports 90c in about the same amount of time, but doesn't seem to stop climbing. Once I get the RTX cards running deep learning loads and the ambient case temp goes up, I don't think there's much margin left. 
 
There are other reasons I'm not crazy about it. 
 
In the running:  EVGA FTWk board, or an ASUS WS board. 
 
I need to run a couple of RTX2080ti or RTXTitans in it. The WS board is x2 slot spacing. I can now buy a 2-slot NVLink bridge, but cooling is an issue. I'm going to need waterblocks, hybrid kits or blower cards. 
 
 

Looking for a board that will fit in a Fractal Design R5 mid-tower. I think a slightly over sized ATX will fit, not sure about the Dark. Will be running 2080ti x2 for deep learning.
 
It's a workstation rather than for gaming, so I'm also using a lot of memory. I think among the EVGA offerings, the FTW-K x299 is more likely.