In some countries the speed limit can change without a explicit sign (speed limits cancelling out at intersections / changes in pavement, etc.). In my experience, in multiple instances the systems offered a speed limit that is higher than the actual one, which can be dangerous if you're just blindly trusting the clanker

The signs also seem to take priority over GPS, I was on a road with a 50mph speed limit tonight and the car read something it thought was a 20mph speed limit sign. I have the beeps disabled but it still displays the red 20mph sign on the dash to let me know it thinks I'm breaking the law.

from the last rental I had, they're not good at that.