I have the ASRock AB350 Pro4, Ryzen 7 1700 and 16 GB (2 x8) Corsair Vengeance LPX 2666. I am very happy with this motherboard and the build in general.
My boot drive is the Plextor - M8Pe NVMe SSD (256 GB) and for data I use the WD Blue M.2 drive (500 GB). Love that the motherboard has two m.2 slots and that the NVMe one supports boot.
Another feature of the motherboard that I really like is the support for fans: 3 PWM case fan headers and a connector for the AMD LED connection for the cooler that comes with the Ryzen 7 1700.
I found overclocking simple and am currently running the Ryzen 7 at 3.6 GHz with 1.20V.
If there is anything else you'd like to know about my experience with the board, I'm happy to comment.
EDIT: just re-read the title and realized that you are talking about the mATX version, whereas I have the ATX version. So perhaps my experience is not identical. I will say that, for my next mATX build, I wouldn't hesitate to go with the AB350M