The videos above are from October. The firmware allowing the full load of GPUs was released in November.
See FAQ/1034921 on the Asus web site. This November FAQ announces the firmware allowing use of all the Gpu slots
Have you tried the latest BIOS v403 Dated 12/15/2017? (With bug fixes to the November drop.) Are you running the November 1 version of the chipset driver?
Are you running the November AMD drivers? Their release by amd seems to be critical.
I can not find a requirement or suggestion to use the p106 GPU in any FAQ or the manual. The manual does show an example configuration with 8 p106 cards but that was only an example that mixed Nvidia and Amd cards are allowed.
The overview has a note stating that until drivers are updated the number of GPUs is limited and that the update is scheduled for 4th qtr 2017.
There are notes in the manual on memory. Asus recommends 2 4GB memory modules for systems with more than 8 GPUs and an initial paging file size 30GB for stability.
There are also operational memory limits. With a 6th gen CPU the 2400Mhz memory will be used as 2133Mhz memory (save money and use 2133MHz memory with 6th generation CPUs
The BIOS memory compatibility list and manual are available at:
https://www.asus.com/us/Motherboards/B250-MINING-EXPERT/HelpDesk_Download/ I would call Asus product support before spending money on a solution that worked for a guy on youtube. It worked for him but may not be the global solution.
https://www.asus.com/us/Motherboards/B250-MINING-EXPERT/