Na página de manual :
-xxx
Show hexadecimal dump of the whole PCI configuration space. It is available only to root as several PCI devices crash when you try to read some parts of the config space (this behavior probably doesn't violate the PCI standard, but it's at least very stupid). However, such devices are rare, so you needn't worry much.
Então, basicamente, é perigoso porque pode travar certos dispositivos PCI, o que geralmente significa que você precisa reinicializar o sistema (e fazer com que você perca dados).