I think maps.me is one of the few apps that are easy to use and contribute to OSM. Unfortunately there was some controversy, because maps got vandalized or filled with trivial/garbage data, because user didn't understand that their notes ended up in a public database.
I deactivate the extension after usage. I also looked at the console/source code and the only external urls the extension is calling are google services (Analytics and Chrome Store) I have analytics blocked in the HOSTS file anyway.
I wish it would be easier for people to contribute to open source projects (like wiki voyage etc) than to Google Places. (it's a bit off topic and I like the Places API, but that's a huge amount of valuable data that contributors are submitting there for free)