Those of you with plans to build an itty-bitty AMD X370 system should head on over to TweakTown and check out their review of the ASRock X370 Gaming-ITX/ac Gaming.
Mini-ITX AM4 motherboards are not that common because many vendors are waiting for AMD's ITX-specific chipsets such as the X300. However, many vendors have jumped the gun and implemented the X370 chipset with its many features because many users want to do mini-ITX based Ryzen builds.