I'm a small-ish time author, but it was really painful for a while since we were all dual-publishing in CJS and ESM, which was a mess. At some point some prominent authors decided to go full-ESM, and basically many of us followed suit.
The fetch() change has been big only for the libraries that did need HTTP requests, otherwise it hasn't been such a huge change. Even in those it's been mostly removing some dependencies, which in a couple of cases resulted in me reducing the library size by 90%, but this is still Node.js where that isn't such a huge deal as it'd have been on the frontend.
Now there's an unresolved one, which is the Node.js streams vs WebStreams, and that is currently a HUGE mess. It's a complex topic on its own, but it's made a lot more complex by having two different streaming standards that are hard to match.
What a dual-publishing nightmare. Someone had to break the stalemate first. 90% size reduction is solid even if Node bundle size isn't as critical. The streams thing sounds messy, though. Two incompatible streaming standards in the same runtime is bound to create headaches.