On the X370 Taichi you have 10 SATA ports, 8 is presumably provided by the X470 chipset, and the remaining 2 by an ASMedia ASM1061 SATA controller.
The 8 native ports seem to work fine, but the ASM1061 might be a complete heap of garbage. Still investigating, but the controller seems to randomly lock up or glitch out when under sustained heavy load, resulting in a dropout of all SATA devices connected to it. Does not seem to cause kernel panics, but this essentially make those 2 SATA ports unusable.
I will update this thread when I can confirm that the problem was indeed caused by the ASM1061 controller, but at this point I have strong suspicion.
PS: fogot to add, but once the ASM1061 locks up, it seems to remain FUBAR until system restart