There really aren't any major benefits/downsides anymore regardless of what you'll read. To clear one thing up, you should compare Rails/Django/Sails (each language's "full" web framework), and Sinatra/Flask/Express (minimalist web frameworks). Each of the full frameworks offer a flavor or configuration of the framework that is focused to building RESTful APIs, and you can build the front end in whatever you want. The only major difference between frameworks in each class will be the language and ecosystem it uses. Of course there will be people that have heavy feelings towards whichever ecosystem they like the most that will tell you it's WAAAYYYY better than the other frameworks, but in reality it's nonsense.

When starting a new project, my major decision is if I want to use the REST API flavor of one of these frameworks + react on the front end (substitute with your front end framework of choice), or build the project fully in the framework itself. I've been writing a lot of python lately, so I go with flask for small or quick/dirty stuff, Django or Django Rest Framework for bigger stuff. React on the front end if it needs to be a SPA.