Hey all, RethinkDB is much slower than MongoDB, see here. Also, the Mongo data-loss problems you may have been fearing have been fixed in Mongo 3.4, and Mongo 3.6 is looking really good, including real-time notifications similar to RethinkDB, and hopefully Mongo 3.4+ lands soon! If you know how, you can spin up your own Mongo (3.6) and set your MONGO_URL to it.
It might be worth reconsidering.