Let me begin by stating that the B350 is supposed to be a mainstream-oriented chipset. It is meant to provide 2017 niceties such as up to 2x NVMe, PCI-e 3.0, SATA 3, SATA Express, RAID support, USB 3.1 Gen 2, minor overclocking capabilities, while still being affordable at the approximately $100 pricepoint.
See AMD slide (Click to show)
Given that at time of writing (March 4 , 2017) the price of a USB 3.1 add-in card is around $20 USD, it stands that the launch-day AsRock B350 motherboards are poor value on top of sparse availability. The price delta to a better designed X370 board with USB 3.1 and better audio is less than $50. In addition, the cheapest ASUS board , the ASUS Prime B350M-A/CSM, has USB 3.1 Gen 2 support and so does the MSI B350M-GAMING-PRO, MSI B350M-MORTAR, MSI B350 TOMAHAWK, GIGABYTE GA-AB350-Gaming, ASUS PRIME B350-PLUS, GIGABYTE GA-AB350M-Gaming 3 , and GIGABYTE GA-AB350-GAMING 3.
Native USB 3.1 Gen 2 as per AMD
Super I/O & fan control
As per http://www.asrock.com/microsite/AM4TSD/
fan control table (Click to show)
AB350M Pro4: no fan control whatsoever on CHA_FAN2
Of these competitor mATX offerings , the GIGABYTE GA-AB350M-Gaming 3 and MSI B350M-MORTAR (a bit higher than MSI B350M-GAMING-PRO on MSI's tiering and actually has 4 memory DIMM support) are available with heatsinked VRM and reasonable specs. The AsRock AB350M Pro4 , even without any detailed overview of the VRM cannot offer the same value as a motherboard due to lack of USB 3.1 , 3200MHz / 3000MHz memory support listed, , extra fan headers, among other things.
Asrock's AB350M Pro4 had very much potential.
Other than USB 3.1 Gen 2, the major upgrade over older chipsets is NVMe for PCie 2.0 x 4 SSDs such as the Samsung 960 Evo / 960 Pro / 950 Pro.
Of all the B350 boards, the only board with the higher quality audio of the ALC1220 is the Gigabyte GA-AB350-GAMING 3 which also happens to carry 5 Hybrid Fan Headers. However the Realtek ALC892 is also a 7.1 surround codec ; with ELNA Audio Caps also (although Nichicon Fine Gold Series or the PCB isolation is not on every board).
Post launch note: The $130 MSI B350 Pro Carbon comes with ALC1220.
Minor Overclocking (it's a B350 board not a X370): As far as VRM goes , provided the inductors / chokes were not skimped on (they appear smaller than the 45A or 60A offerings on the X370 boards) the mosfets should be able to handle 25-30A per phase for the board to be competitive with the GIgabyte and ASUS offerings (those boards have 4 phases so realistically based on the 4C09B / 4C10N / 4C06N Low RDS(on) mosfets used they can deal with about 120-150A at most based on 6W power dissipation). The Asrock B350M Pro4 for example touts 42A power chokes.
However, hardwareluxx's VRM thread lists the Asrock B350 boards as using PK618BA (NIKOS / UNIKC) for the lowside mosfets so even doubled up to 6 CPU phases this means it is thermally limited.
PK618 datasheet: http://products.niko-sem.com/images/product/138874237504741113.pdf --- footnote reads "Package limitation current is 26A"
The Gigabyte B350 boards apparently run 2 low side 4C06N per phase & a 4C10N high side mosfet per phase with 4 CPU phases
4C06N datasheet: http://www.onsemi.com/pub/Collateral/NTMFS4C06N-D.PDF
4C10N datasheet http://www.onsemi.com/pub/Collateral/NTMFS4C10N-D.PDF
See also buildzoid's overview of the Asrock B350 Pro4 (https://www.youtube.com/watch?v=KzRG_LjUl8Q) in which he suggests it's actually 3 effective phases running two mosfets per phase without doublers. The board shown has the SM4337 and SM4336 mosfets though.
As for capacitors, which are the other major failure point in power delivery:
B350 Fatal1ty K4 appears to use a total of 4,100μF for the CPU which is respectable but not spectacular.
2.5V 820 FX
5 x 820μF = 4,100μF
The Asrock B350 microatx boards do not use 5 of the capacitors but only 4 (I am unsure of the capacitance on those). Meanwhile the B350M Mortar from MSI uses "5KW12 560 6.3V" capacitors , 7 of them would be 3,920μF.
PCie slot reinforcement , as Ryzen does not have an integrated GPU
What MSI gets right is the addition of the steel PCI-e reinforcement for graphics cards across their entire line ; ASUS boards without the steel reinforcement incorporate a reinforced plastic design.
Gigabyte's GA-AB350-GAMING 3 does have a steel reinforcement for the PCi-e slot as does the Asrock Fatal1ty AB350 Gaming K4.
* disappointed (without the extra s)