Yes, in general they are working, but most are not compatible with OpenFirmware, only with EFI on MacPro. So it will be detected and will work on G5 but booting operating system won't be possible without a proper OpenFirmware support.
Hi,
I've been searching whole internet for PCIe card for my G5 Quad Late 2005, I've just noticed that only SATA II interface card exist for this system it's https://www.firmtek.com/seritek/seritek-2me4-e/spec/ and supports booting from attached drive. Maybe there is more unofficial SATA II/III...