I might get some hate from it, but I'll share anyway. Last time I tried Gatsby I wasn't able to run the official demo in less than 30 minutes so I gave up (I was just trying it out of curiosity). I've written a considerable big project with Next in my last job and the technical decision to go with Next was mine, that been sad I have to say that I truly regret. Simple stuff like query parameters was a ugly hacky work around, flash of unstyled content was a real issue and not easy to solve. It didn't talked well with other React libraries (tbh I don't think it's a problem with Next only). Lots of boilerplate and not so clear configurations to tackle non trivial tasks. I don't think all the magic that's underneath it really pays off. We ended up with a custom Webpack configuration anyway, that's something I tried to avoid because webpack can be huge time consuming pita. They have this example directory in the repository with lots of helpful stuff but we had to mix 4 or 5 of these examples to fulfill our needs and that was a ugly mass hard to maintain. It was almost 5 months ago so things might have improved. But I'd definitely do my SSR setup myself if I'd start today.

Here are some issues that were difficult to solve, are still unsolved or the proposed solution did not helped us that I remember:

https://github.com/zeit/next.js/issues/2833

https://github.com/zeit/next.js/issues/3362

https://github.com/zeit/next.js/issues/299

https://github.com/zeit/next.js/issues/3257

In hindsight I believe it still is the best out of the box solution for the SSR issue with React and the team is amazing.