I don't understand the "right amount of control" bit, there are plenty of options to control what data moves to and fro in NextJS, but the CEO of Vercel has replied to you in a sibling comment; he's going to be able to address your technical concerns better than me.

I'll just add that I'm not familiar with either of those sites, because I'm sitting in Spain and neither operate here, but on first inspection the UX has felt brilliant to me - particularly Walmart's. Everything loads very fast, the search is great, etc. I cannot say if their code or eng practices are a tire fire but the product looks good to me.