Earlier quoted context omitted.
Somewhat an outsider (but I have used Netlify and eleventy). It's basically a tale as old as time: trying to build a sustainable business around open source technology and perhaps fleeting trends in software development. Vercel and Netlify offer essentially the same thing: easy hosting of mostly static websites + a few bells and whistles. They each support development of frameworks (eleventy, Next.js, Gatsby, and per…
One big thing is that Vercel owns Nextjs and has started to do direct integrations with their platform. 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.
It was my (perhaps outdated) experience that in fact some elements of JS (like using scraper libraries) would not work on Vercel at the time due to memory of serverless function and other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.