You're damn right that 6 rear USB ports are too few. Asrock should've moved the pair under the PS/2 port to under the second NIC port, and replaced them with a pair of USB 2.0 for keyboard and mouse. Don't know why they're being so stingy, especially at this price point... it's not like Z370 suddenly no longer supports the older standard.
Then they go and put three USB 2.0 headers on the board. Don't get me wrong, I appreciate the vast options regarding front panel connectivity this board brings, but how many people are going to have 1x USB 3.1 Gen2, 4x USB 3.1 Gen1, and 6x USB 2.0 ports on the front of their case?
I'll give Asrock the benefit of the doubt here and assume they're trying to move away from USB 2.0 entirely, which would be great, but they could've at least added a switch of some kind to e.g. bifurcate the front 3.1 Gen2 port into an additional pair of 3.1 Gen1 ports for the rear if the front header isn't in use.
Review errata: first page has "Storage: 8x SATA 6 Gb/s port (Intel X299)", I think you meant something like "8x SATA 6 Gb/s port (6 from Intel Z370, 2 from Asmedia ASM1061)". The "Storage Interfaces" page does have the correct info.