Still, the two buckets I described above hold true. If someone is returning to your web app - Slack, or Trello, for example - you don't care as much about SEO because most of your visitors are return visitors. Your splash page, on the other hand, needs to be optimized for SEO but won't have much interactive complexity, so it doesn't need client-side rendering.

A lot of seo data is meta tags in the head. These aren’t going to render client side.