Google Maps is cheap (in fact, it's almost free compared to any non-self-hosted OSM solution), but it suffers from inferior performance in their JavaScript engine when lots of icons are involved (100+ resources generate serious lags), and as we know from our pre-1.13 version their native SDK is completely unstable on certain older phones.
Additionally, Google Map's tooltip/info windows on land markers aren't as easy to customize or add buttons on them (and I think we're all used to and love the additional controls available for each land).
The map can only be pre-loaded on a phone until a certain zoom level or if you are playing in a specific region only, otherwise it's going to take lots of space (and I am quite proud of the fact that "Domination: Earth" weighs only around 15MB for Android 6+ and iOS).
Because of the fact that we allow to zoom in pretty close on any part of the world all of the associated map tiles can't be pre-loaded, I'm afraid!
Don't worry, once I finish building our own map server things should work pretty smooth and look good; plus I am personally attracted by the possibility of not depending on any map provider (Google Maps or other)!