On top of that, until the past couple years, there were only a few exotic ARM builds (probably mostly in custom or enterprise land) that even had standard PCIe slots, so many vendors didn't (and still don't) test their drivers—even if compiled for ARM—on any real hardware.

I hope this changes, especially since Macs now have ARM64, the Pi CM4 supports it, and some other inexpensive SBCs have 4x or even 8x lanes on the more expensive ones (is there an ARM motherboard out there with a 16x lane bus?).

For the Pi in particular, until the CM4 came out, very few people would ever get access to the PCIe bus on the regular Pi 4 model B, so only enough work was done on the kernel side to ensure the VL805 USB controller worked, and a few bits were just never fully implemented or tested (e.g. BAR allocations, MSI-X support, some memory access functions).