$500 mainboard, $1k chip, another $400 or so for ram. Costs a chunk to set up into one. For 1440p or 4k gaming it's no better than my current 9900KS. With the demise of SLI there's less incentive to get more PCIE lanes, too.
That Asrock ITX X299 sure is a neat little board. Did you get the monoblock to keep the VRMs from going supernova?
I have no problems with [email protected] on the ITX mobo without additional airflow. I had it in multiple rigs but never in my daily rig. Power design is pretty good and people were testing it with 18 core CPUs too but I don't remember results. I also couldn't get the monoblock and when it appeared in stores then it cost $300. I skipped it as I had the motherboard for over a year with Watercool block.
In Poland you can get used X299 motherboard for $150 with warranty and still no one is buying them. That's why I see no point in selling mine (I have ASRock X299 Creator and EVGA X299 Dark too). On the other hand, the cheapest 10980XE costs ~$1.1k here, what is too much for me so I stay with what I already have.