> Atheros 5000 based PCI cards won't work on G5 machines?

Not with MorphOS 3.2 apparently:

https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=9234&forum=3&start=16

> Don't they have compatible PCI bus or why?

I don't know what the reason for reported non-support of PCI slots in PowerMac G5 is.