You can technically use Mongo and Node on the backend and choose to use whatever you want on the front-end. You don't need to limit yourself to MEAN or MERN, they're just a bunch of things which work together or perhaps, often used together.
Like what can I do with node+react that I can't do with just node - Let me address this now. NodeJS is a run time to write server side JS code, while React is a library to build your UI.