I started signing my cards with an all caps "ASK FOR PHOTO ID" 30 years ago. It raised a few questions when I would travel to the US and use them there, but was never refused a transaction.
In fact, even verifying the signature is no longer required in at least the US.
Signature verification also only solves cardholder authentication, not card authorization (i.e. figuring out if the card is funded, still valid etc.)