I feel like there's some confusion over peripheral support, and host support. What can I do with a serial port? Everything. I can talk to every device that ever used a serial connection. Sure, I might need to write a driver, but that's really a separate issue. There's absolutely no problem with "your serial port does not support <gibberish> so you can't use this particular mouse/printer/random industrial equipment".

With your PCIe example, again - I don't expect every card to do everything. But I do expect every card to work on my computer's PCIe bus.