btw. i'm not claiming Next.js and Gatsby are misadvertising themselves, but instead i see lots of people categorising both frameworks as such, which is wrong
i'd like to emphasise that while both frameworks have their uses, they are not static site generators and suck terribly when you really need to generate more than a bunch of pages
Large production instances of Next.js tend to rely on server-side rendering almost exclusively. Paired with react-query + cached api/Redis, you get short build times and a fast user experience (Remix has a very similar approach as well).
Gatsby is also a great static site generator. Most sites render with JavaScript disabled + the framework provides some of the best tools to create increase Lighthouse scores. It's only weakness is the build speed once you pass +100 pages.
That said, the primary issues around static site generators are:
i'd like to emphasise that while both frameworks have their uses, they are not static site generators and suck terribly when you really need to generate more than a bunch of pages