There's also the I/O ports (PCI has three address spaces: configuration, memory, and I/O, with the I/O space being a legacy x86 thing which does not exist on AR

There's also the I/O ports (PCI has three address spaces: configuration, memory, and I/O, with the I/O space being a legacy x86 thing which does not exist on ARM and most other CPU architectures), which some cards might require; and the PCI address window (BAR space), which might be too small for some cards (usually GPUs; see for instance https://www.jeffgeerling.com/blog/2020/external-gpus-and-raspberry-pi-compute-module-4 https://www.jeffgeerling.com/blog/2020/external-gpus-and-ras... for a couple of examples where it was too small.