Unless you MUST have the absolute best SEO, I'm not sure that it really matters having SSR for most people/apps. I don't know how much traffic is really driven from search engines depending on the site, and I really don't know how much you get penalized for having an SPA anymore. I mean if your initial load is under 2-3s and in-site navigation is fast, you'll probably be fine.
The past few sites/apps I've consulted on got the majority of their traffic from outside SEO.
More relevant metrics are TTFB (time to first byte) and TTI (time to interactive).
I wouldn't want to implement a marketing site in express/pug. I want the flexibility of components and all the other benefits that React offers.
Me too. But if you ever did marketing landing pages for products you might know that one landing page is not enough. You need thousands which match to Ad Word campaigns or which are optimized to different SEO fields, domains and whatever. While you still could manage this with Next and nice components, Next's subpar routing might be an obstacle.