Maybe 40% of my city is missing (top-20 in the US) including absolutely everything in my neighborhood other than a couple parks and churches. It gets my address to the correct block (between X and Y Aves) but that's it. Some small towns around me barely exist, others are absolutely complete.
You'll probably do ok in SF/NYC/LA/Chicago etc, but it gets really spotty beyond that.
OSM is becoming pretty impressive too. Even smaller towns here in Sweden are decently mapped. A huge difference compared to just five years ago¹, far greater difference than since Apple Maps was launched as far as I can tell. Apple Maps launched in a better state, yes, but not in a good state, and I think it's been moving slowly since then. OSM launched in a minimal state but are moving forward like a maglev train.
¹ Also visualized here: https://labs.mapbox.com/ten-years-openstreetmap/