Definitely my biggest concern working with nextjs so far is that the line between when something is will execute server side and when it will execute client side is a bit blurry until you test them. It's manageable but requires some trial and error IME.
That was problematic for me when I was testing it out. Can’t access the window object without excessive workarounds that I couldn’t make work with my project. I ended up reusing existing react components and move it to all browser rendered react and even that experience has me leaning toward moving it to Vue or dropping the framework altogether and grinding everything out in css again. That said, I did enjoy using styled components. I found that pattern pleasant to reason around.
