Quick report for anybody following this exact build:
Up and running for over two months now. Not one single kernel panic, failure to boot, crash or any bad behaviour - rock solid, and have transitioned away from my ageing MacBook Pro to make this my main Mac and work computer. Use the Mac 8hrs+ per day for all general office productivity, (and outside of work, Windows for games and VR).
Few notes:
Using Nvidia 1060 6GB , running duel display 4K and 1080p. Multi-boot system (Win 10 on NVMe and macOS 10.13.4 on SSD, each with a separate media HDD). FileVault 2 is on. Messenger, Mail, iCloud, App Store etc. all in daily use and seem to be as per native. Using Carbon Copy Cloner to clone disk to internal backup drive, just in case - very useful for troubleshooting when first building and tweaking. No WiFi hardware added so use Ethernet only.
Have NOT bothered to try to fix/enable:
HDMI audio; USB 3 seems to be working as USB 2; loading bar colour change when booting with FileVault on (effects nothing); have not enabled full continuity and handoff (but do have a GMYL Bluetooth dongle with the Broadcom BCM20702 chipset - Apple wireless keyboard works out of the box). Booting with the 4K AND 1080p monitors attached causes a black screen/no monitor output hang after entering login password - I just keep the 4K powered off until booted to desktop, then switch on, automatically detects the second monitor with no problems.
Hope that helps anybody considering this build.
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Edit -- since I was passing by anyway.... 25th November 2018
Still using the same build (now with GTX 1080), remained on 10.13.4. Has been rock solid. This is my main computer ~ 8 hours use per day. Never a single kernel panic, apps crash no more frequently than they did on my real (laptop) Mac --- stability has been better than I could've hoped for with no problems (unless I've tried to fix something that wasn't broken
).