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Regarding the VRM's. You technically said it right, it appears to be an 8+4 design, but it's not.
It's a 4+2 (IR35201), with each phase having 2 MOSFETs since it doesn't appear to have phase doublers.
^at least that's how I see it, someone correct me if I'm wrong please.
Seems a step down from the Taichi no?
For Asrock X370 boards, the ranking as follows:
X370 Professional Gaming
X370 Taichi
Fatal1ty X370 Gaming K4
X370 Killer SLI
Wendell is correct.
This particular board is using the IR35201 8+0 phase flex mode pwm,
which on this board runs in a 4+2 phase configuration.
4 phases being doubled to 8 phases using 2x IR3598 doubler / dual driver
and +2 phases being doubled to 4 for soc voltage.
Hm ok thanks then the info I read is incorrect I guess 
The IR3598 is a special doubler / driver.
it can either be used as a doubler, but it can also be used as a dual driver.
Which on this board its used as a dual driver..
The out come is basiclly still the same.
THe IR3598 basiclly takes 2 pwm signals in, and provides dual driver outputs.
Which basiclly means that you can create 8 phases with just 2 dual drivers.
Because each signal get its own set of power components.
I think this is the best way i can explain it.
These boards basiclly have the same vrm implementations.
They are indeed a significant step down from their bigger brothers the Taichi and Professional gaming.
But never the less the vrm implementations are basiclly fine in most cases.
They just have less phases, and the mosfets are cheaper and have less current capabilities.
Its just a bit less efficient overall.
But thats why those boards are midrange afterall.
Still great to see that they still offer the IR pwm, which means great voltage control.
The Taichi and professional gaming really use top class components as far as the vrm circuitry is concerned.
You cannot find anything better then the X370 Taichi or X370 professional gaming atm.
It is hard to beat the price aswell.
Yeah the Taichi in particular is just great value for money.
I can recommend it to any OC enthusiast who wants to get the max out of their R7 chips.
But never the less, for people who cannot afford those higherend boards.
The Asrock X370 Killer Sli or the Asrock X370 Fatal1ty gaming K4 are,
definitelly not a bad middle of the road choice.
Writing this from a system based on that board^^ https://forum.level1techs.com/t/new-horyzen-datacap-warning/114708
I place it equal on the chart with taichi .. my reasoning same overall board design.. the additive to the professional gaming seems to be all the gamer bling which honestly is usually buggy and shitty and i havent a need for it as a gamer but thanks for clarifying
I have/had this motherboard, as for the BIOS/UEFI updates...They have 3 methods to update.
The reason I say have/had this motherboard, I updated the UEFI through Windows at version 2.0...it started running, my screen went black, and the system rebooted....MID UPDATE. So it bricked my board. ASRock did RMA it, but when I got my RMA board...it was DOA and damaged. So I am waiting on the second RMA to get this system back up and running.
The Taichi and Professional gaming from a build quality standpoint are pretty much the same.
They have the same vrm components and all that stuff.
The main difference is mainlly a few feutures.
yeah and you know my feelings about overloaded with features and bugginess cough asus cough
Yeah it really depends on a persons needs basiclly.
Asrock doesnt really overbloat their boards too much.
The Taichi is a very good example of a powerhouse board with all the base feutures,
and functionality that a general user needs for a good price.
Well to be blunt ive never found a use for the "gaming" features on boards. Have you found any useful?
Not really nope.
Main thing i care about is vrm implementation and overall build quality,
UEFI and just the basic feutures i need.
I dont really care for gaming feutures or flashy RGB leds.
However i can be picky about certain design flaws on a board.
actually this is ONE feature I might be catching onto when its done elegantly
You ... picky... you dont say hahah 
ehh dont worry im probably more picky
I was wondering if this board supported USB 3.1 gen 2 speeds in the rear i/o since every website specifications state that the only has usb 3.0. Even though it looks like it should be USB 3.1, and almost every board this generation seems to have it, this left me wondering if they left that feature out on this MB.
I'm thinking about building my first PC with either this MB or the Asus prime-pro and was curious about this since Wendell said in the video that it is 3.1 gen 2.
Any thing else I should know about these MBs would be usefull.