system-ui is not suitable for general web content. Apps might have an argument for using it, but general web content should not use it. https://github.com/w3c/csswg-drafts/issues/3658 has some details (mostly by others, but I chip in towards the end), and I’ve written more in other threads here on HN too which you can search for.

(That `box-sizing: content-box` is a good default might be my most unpopular CSS opinion, but writing off system-ui is probably not too far off it. But I care more about it because it’s one that actually affects users, not just developers. Maybe I should get round to pestering browser makers to improve their defaults for the sans-serif generic family so that “proxy for a maybe-prettier sans-serif” goes away as a reason to use system-ui.)