It's actually pretty good in some quite remote places, but like all mapping the coverage does vary (see examples elsewhere in this thread of remote places). Probably some countries they'd have to either fill in with commercial data, or sponsor efforts to add data to OSM.
The main advantage would be that OSM has all the adding/editing/updating stuff down - that is a major operation and if Apple are trying to do it all themselves while merging inconsistent data from several sources and taking care of customer complaints they'll find themselves falling farther and farther behind people like Google who have devoted years and years of effort and a lot of money to this just to get where they are now.
Perhaps Apple considered OSM as a basis and decided it was just too limited in areas that were important to them - it would be fascinating to find out about their internal tools for this and where the data came from but I suppose we'll never know.
The main advantage would be that OSM has all the adding/editing/updating stuff down - that is a major operation and if Apple are trying to do it all themselves while merging inconsistent data from several sources and taking care of customer complaints they'll find themselves falling farther and farther behind people like Google who have devoted years and years of effort and a lot of money to this just to get where they are now.
Perhaps Apple considered OSM as a basis and decided it was just too limited in areas that were important to them - it would be fascinating to find out about their internal tools for this and where the data came from but I suppose we'll never know.