Drove a rented Kia in Central Australia recently that had this. It was fine: just stick below the speed limit. "Dangerous" to someone who continuously rides the speed limit perhaps.
I own one, unfortunately it is often wrong about what the speed limit is. So it’s not quite so straightforward.
We’ve a fairly new car with a similar system (that can be ignored/disabled thankfully). It seems to have both a some GPS/location based awareness of what the speed is meant to be that it falls back to as well as a camera based detection. I find it useful as a reference incase I don’t know what the speed is and missed the previous sign, but it’s also regularly wrong and not to be entirely trusted. For example the on ramp to the closest freeway near us it consistently reports as 10km/h when it’s actually 100km/h. I don’t know what it is about that sign specifically it can’t read as it looks fine to the human eye, but it would try to cap our speed on the freeway to >=90 km/h slower than the rest of the traffic (most are traveling slightly above) if we were to have the system enabled. As others in the comments have suggested it also can’t be trusted to get the speed right in school zones or for the correct distance for variable recommendations (e.g., speed bumps, roundabouts, etc.)
The worst in my previous car was trucks bearing stickers indicating their max speed was 80 kph, and the car spotting this as the new limit and beeping me for leisurely passing at 120...
The speed limit is the speed you should be driving in most circumstances, and is set far below the maximum safe speed for the road. Driving significantly below the pasted speed is as dangerous driving significantly above it.
Some roads have ridiculously low posted limits. Everyone goes well over, yet those roads are not noted for accidents. If they set the speed limit according to how everyone drives there, then I'd be in more in favor of alerting those who go even 1 km/h over that.
Driving significantly below the pasted speed Without good reason will get you failed in the UK driving test for not making adequate progress.
As my office colleague found out recently.