I've got a new Z370 board which works find to passthrough a GPU in PCIEx16 slot one without the ACS patch, however if I want to use any PCIe device in slot 2, it shares an IOMMU group with slot 1 and so to separate, I need to use ACS override. I haven't noticed any ill effects by doing this, but I haven't ran it for very long. Given the warnings I've seen, should I be concerned long term? Why are these warnings there?

It might work reliable bu there could also be problems difficult to detect, for example:

https://forums.unraid.net/topic/37959-guide-passing-through-network-controllers-to-unraid-6-virtual-machines/?do=findComment&comment=602127