Its extremely expensive, and I'm aware of two companies migrating away from NextJS itself (neither were ever hosting with Vercel, though).

I've only talked in depth with one of them, but the reason is complexity: they want "a react framework" and every update to NextJS substantially raises the complexity bar, introducing a large number of features that they don't want or need. There's also been at least one incident of a NextJS bug which only affected non-Vercel deployments; and that incident came up in this discussion as evidence for their concern that NextJS doesn't have a future outside of Vercel (the company and the platform).