SSR is "fine", but it doesn't need to be this painful. Here's an example buggy piece of code:

  {{range $i, $file := .Diff.Files}}
    <div>
    {{ if $file.IsSpecialFile }}
        <div>Special file ({{$file.name}})
    {{ end }}
    </div>
  {{end}}

Because of the missing </div>, instead of a list of files, it produces a file listing inside of another file listing inside of another, etc. That's just something you'd never be able to accidentally do in NextJS, or anything that uses JSX-like structure.

Then there are clerical things like where to put styles and presentation logic. Gitea uses a mix between jQuery and Vue, with CSS rules sprinkled in randomly. The result is that by looking at the template and the class names on each element, you have no idea whether a class name or ID is going to have significance to jQuery, CSS, or if it's just dead code. Something like styled-components, and use of a single stack (Next as opposed to jQuery+Vue) eliminates the fear and clerical aspect of development.