1. Maps.me and Organic Maps were initially designed for an offline experience, so they don't even try use data when I have a spotty connection. Unless you explicitly disable data, Google Maps will always try to load new tiles. This can be annoying when your reception is weak and Google renders fuzzy tiles instead of falling back on the already downloaded tiles.

2. You can't download offline maps for South Korea in Google Maps. There may be other areas in the world like this.

3. OSM data is better for hiking trails in my experience.