Pros:
- Additional documentation on top of what's already available for the individual components, and how they work best together
- You don't have to make as many choices, which can get fatiguing and you're more likely to get them wrong
- Sometimes these groupings help with employment and hiring, like what LAMP did.
- I love React.
- I love React.
Cons
- Redux being included as part of it contributes to the issue of people bringing in Redux too early and/or in apps that don't need it.
- MongoDB might not be the best choice for every use case. It also hasn't had the best rep recently with the accidental exposure/deletion/ransom of people's DBs.
All in all, I think MERN is a fair guide for those who just want a more complete paved road. To be honest though, I don't know anyone personally who has used the actual MERN stack CLI or boilerplate stuff. They just use the things that happen to be part of the acronym.