I've been working on React projects lately and also spent a lot of time gluing together all the different libraries and setting up a proper build process via Webpack. I've also worked on Rails projects in the past, and I have to say that I prefer the modularity of being able to structure my projects according to the problem they're trying to solve. For instance, you wouldn't create a single-page app in Rails.

Also, with regards to Webpack, even though it's complex, it's extremely powerful, more so than anything else I've found. It includes many ways to optimize asset bundles considerably, by splitting them up into different files, removing unused code, and of course all the usual stuff like minimizing and gzipping the assets. In comparison to Rails it also compiles way faster, by only recompiling the code that you changed. It also includes many development tools, like an auto-reloading server and even updates your webpage without a refresh (HMR). All these are things that just aren't possible in Rails without much, much more effort.

Edit: I guess there's a first for everything, but I'm really confused as to why this was downvoted. If something I said is wrong or debatable, I'd be happy to hear it and discuss it.