Doesn't need to work everywhere for it to be useful. You can always fail fast, literally. The data is available for wide swathes of Europe, for example. Last time I checked it was very accurate even for temporary construction sites.

Variable speed roads, either make the limit the highest speed that's in the range, or come up with a way to communicate the current limit. Most new cars are always connected, anyways. And again, you can fail fast, at least initially. As with all driving aids, the person in the driver's seat bears the ultimate responsibility.