Post author here. I've been developing in Ruby and Rails for almost 20 years. Here are some of the downsides in my opinion.

- The Global interpreter lock (GIL) in Ruby is less performant than async thread programming in JS (and some other languages)

- Rails creates a monolith rather than a bunch of independent endpoints. If you have a large team, this can be tricky (but is great for smaller teams who want to move fast)

- How Rails integrates with JS/CSS is always changing. I recommend using Vite instead of the asset pipeline, unless you're going with the stand Rails stimulus js setup.

- Deploying Rails in a way that auto-scales the way serverless functions can is tricky. Their favored deployment is to server of set size using Kamal.