Although the features can work without Vercel, it's a second class citizen.
The more popular Next is, the less reason people would choose netlify over nextjs as a default option.
So there is frustration that the React project is now perhaps an extension of Vercel's interests, and focused on helping them sell hosting.
Never ending drama in VC land.
It was my (perhaps outdated) experience that in fact some elements of JS (like using scraper libraries) would not work on Vercel at the time due to memory of serverless function and other limits. But locally they worked on my dev machine.