Btw I was looking into Django recently, they are still using the “older” paradigm you are referring to.
Then the SEO penalty is just the increased page size (maybe some extra loading time). But at least the content can be crawled normally.
If you're twitter or facebook, or have enough domain reputation already that your crawl budget is big enough to offset, then it doesn't matter. If you are not already a big deal, then it takes a lot of off-site traction to overcome.
Practically speaking, for two otherwise-even sites just starting out, a statically rendered site has a pretty significant SEO advantage over a JS-only site.