I feel like the elephant in the room here is that their back end was in RoR before Next.js and remained that way the entire time. They then switched from next.js to a framework designed, in part, with RoR in mind. It seems unsurprising that they had a much better experience using a thing that was tailored to their use case.

I love Next.js. I have used other frameworks including RoR and there is nothing like it (except Svelte or Nuxt but I view them as different flavors of the same core idea). But I only use it to make monoliths. I can see getting frustrated with it if I was using it alongside a different back end.