You can't really brick GPUs, at least I failed to do so trying to flash the wrong BIOS or even when I restarted a frozen rig mid BIOS update. But then again I only ever used the DOS version of NVFlash (which you can use to list your cards and also you have to specify which card you want to flash).
Can you be so kind then to tell me how can I make my PC recognize all of my 280x? I got 5 of them that are completely bricked/dead (No signs of getting detected by the OS). When I look at device manager it only says standard VGA and it won't let me install drivers on them.
I can't really help you with AMD cards, your motherboard might not have enough resources (lanes) to run all cards or it could be a driver issue or something specific to AMD, I don't know.
Tried them with multiple motherboards, risers, PSUs, drivers, OSs. They are just dead. A curious fact is that it does give you video if you attach an HDMI cable to it.