When you make a jump to a JS stack (whether client side or server side) you have a lot of options but no best practices or conventions to follow. You have to make a lot of choices which have already been made for you in RoR. Even though SPA frontend dev has settled between React and Vue, you still have to make a lot of decisions. React also bills itself as a view library. You have to make decisions about state management (if needed), routing etc etc. A vast chunk of these decisions are made for you in RoR so that you just work on the app. I think this is what the author was lamenting about.
If the platform is too unopinionated you will generally wind up with: 1) A project with many competing ideas (most js projects I see end up like this) Or 2) A team with some people feeling like they are not heard
These days I prefer building something up from microframeworks and I like to take the opposite side of the trade-offs you mention in your comment. There are a lot of aspects of Rails I ended up really disliking over time.
But at the same time, I still find myself thinking back to some of the pleasantries that Rails gives you and wishing I had them in my bespoke applications. Even just things like being able to enumerate your routes from the CLI or seeing your SQL queries and query response time in the terminal during development. If it's something you have to implement yourself, you just tend to not do it at all. And there's a lot a framework can do for you with some opinionatedness.
I think the classic example of this realization is when you deep dive on microframeworks, accumulate them in production, and encounter things like performance issues but realize you don't even have the tools to debug them because you didn't build them.
From this hot take on JS, I'd think people are suggesting that the development experience of all the other available clients are a lot better, but then I became an Android and iOS developer and that's not true either.
Swift isn't bad, but the language isn't the only thing that dictates how hard client development is. I'd rather use React than UIKit abstractions, that's for sure. And CoreData is probably the worst and most outdated abstraction I've used across all platforms. And even if I think iOS development is the ideal way to write software... it only runs on iPhones and iPads.
I mean, of course the Rails side of your application is probably nicer than the client side where you get to stay in your simple request -> response realm that runs on the same machine. If given the option, I bet almost every HNer would rather be the person paid the same to build the API endpoints vs the crapshoot that is client and UI development.
But I see this perpetually confused as Javascript vs <my favorite language I use on the server>. For example, Ruby fares pretty poorly against Node if you forget the client, like Sinatra vs Express/Koa.