Neither can Google maps. Many countries have official maps, some are even open data, but even those don't have perfect coverage of "unofficial" features like footpaths.

Here in the Czech Republic, a combination of OSM and official data (ZABAGED) is the best bet, somewhere else it could be Google's data, if they made it available.

It depends where you're looking at. In US, I think google would do pretty well - better than OSM, for I.e this ATM question. Obviously, no maps are always 100% accurate.

True, but that's kind of a chicken-egg problem. If OSM were more popular the resulting influx of contributors and other resources would solve the data quality problem pretty quickly.