I guess it depends on what you mean by reliability - from the FAQ:
RethinkDB is not a good choice if you need full ACID support or strong schema enforcement — in this case you are better off using a relational database such as MySQL.
No.. I am not talking about full ACID support. Hardly any of the new object stores like MongoDB guarantee that in all conditions. I am talking about being generally reliable to store data and fetch data for a relatively important application on production.
RethinkDB is not a good choice if you need full ACID support or strong schema enforcement — in this case you are better off using a relational database such as MySQL.