While Microsoft rarely pursues legal action against individual users, the consequences can be technical: your activation may be randomly invalidated by a future update, leaving your system in an unactivated state. The more immediate and serious consequence is exposing your personal computer and data to potential malware attacks.
Legitimate software comes with support, security patches, and updates. Pirated software does not. If your system breaks due to the activator, you are on your own. Are There "Safe" Versions of Ratiborus Tools? kms tools ratiborus