There is no such thing as a barrier in WIN10. That used to be implemented in the drivers. You need upto date win10 (1709), latest AMD drivers (18.2.3 Feb. 2018) - install ONLY driver!, compute mode switcher (
https://bitcointalk.org/index.php?topic=2815803.0), a max number of 13 similar cards and a mainboard with enough PCIE slots. Then you're good to go. My rig with 13 cards works flawlessly and the output is close to 400 mh/s on ETH/nanopool. no probs at all.
The 13 card barrier is because of the BIOS of my mainboard (Asus B250 expert mining). Power draw on the wall is 1750 watt for the rig including monitor (my rig is like an open cube built with aluminum profiles, with wheels and inside with 3 rows of cards on top of each other).
So after reading through everything, it would appear the problem lies with the AMD driver. I am using the AMD blockchain driver, and apparently that will not work with more than 8 GPUs (cause Windows to not boot up, kind of dumb given that mining is the only reason why multiple GPUs are needed in the first place.). Guess I have to uninstall the AMD blockchain driver and re-install the 18.3.1 (there is no 18.2.3, it goes from 12.8.1 to 12.3.1)standard driver (and switch to compute), and see if this works.