From memory it was mostly locked scrolling, slow page loads, awful rendered video, etc. Ironically i recently saw Google with the same problem on their new Phone product page (a few months back).
It seems Designers are being given too much freedom with completely no concern for practical implementation. No one on the dev side stands up and says "but that'll lag like mad" - they just implement the bloated slow mess to spec; and god knows performance is never in spec.
So yea, i might agree with the GP comment - i do prefer "modern" look over that of Sourcehuts.. if it worked. But as it stands, Sourcehut's UI is faster than every modern UI i deal with, and i want things that work - first and foremost. Fast.
Modern pages suffer here.
That said, you can have a "modern" UI (although not UX) from SourceHut with only CSS changes. You can write all the CSS needed for a modern UI without doing it wrong like Apple did, and (if you know what you're doing) without adding megabytes of bloat. Add caching to the mix, and SourceHut would be just as fast but look more modern.