Many people praise MERN for its simplicity, flexible deployment options, and its ability to be used as a boilerplate code generator. However, others criticize MERN for its lack of maturity, large learning curve, and steep curve in terms of maintenance overhead.
While MEAN has been around in the JavaScript community for a longer time, it has also faced criticism for its lack of clear documentation, steep learning curve, and overall complexity.
When choosing between MEAN and MERN, the best way to decide would be to evaluate both frameworks on their pros and cons and make a judgement call on which one you think is better in that given area.