There is nothing new, If you read about SOA or micro-services based applications you will see similar patterns where a part of system is coded in different language and other part in different communicating over message queue or HTTP calls.
The only difference between MEAN & MERN is the only front-end framework used Angular OR React (Selecting one of them is really a hard choice, you will find a lot of debates over Angular VS React). My personal preference is Angular but, many may argue for react.
Learning Node & Express is pretty straight and you will start working with them within days.