I do not agree with that assessment at all. Next.js is great for making SPAs and full web apps. I've made three (profitable) projects with Next.js and never felt like I had to fight it
What we needed was convention to deliver static content easily without needing to build it ourselves. But we also needed it to stay out of the way of our business logic. Overall, Next has done a stellar job of that. It allows us to build the stuff that matters to us while leaving what Next is great at to Next and its maintainers.
The vercel platform has also been great for us when staging deployments. It's a little slow, but the integration and ease of use again lets us focus on our own business. I don't care to build out staging infrastructure for mostly static content. It's well within everyone's ability on this team, but it's time consuming in investment and maintenance. Recouping that cost is what Next has been great for, from end to end.
However, if you build an application where that browser-as-state-machine doesn't hold up, you're better off somewhere else.
React is kinda weird… you have to do subcomponents the "React way" and I think a lot of folks struggle with that; it can feel like fighting. But I don't think Next makes that any easier or harder.
I don't know what you mean by "nested routes." Routes are just URLs that map to files. You can create a directory with files, and, uh, subdirectories with files, if you want…?