You may happen to like how system-ui looks on your system (macOS? Windows?); but do you have any idea what it looks like for the person reading your resume, perhaps on Linux with a custom theme, or on an Android device where the vendor chose to ship some brand-specific UI font? Or on a 5-yr old version of the OS, or on the version that'll be current 5 years in the future?
IMO system-ui is hardly ever a wise choice for content.
> Android device where the vendor chose to ship some brand-specific UI font
Chrome (and Android Webkit) hardcodes Roboto as the system-ui font (in such a way that both Samsung and Xiaomi devices (which both have font options) cannot touch), and Firefox brings its own font rendering which also hardcodes Roboto.
If the system-ui font of Linux is set to a silly one (according to your opinion), then the user-in-question would realise that they are the one who picked that font that you called silly, in other words they don't care about your opinion at all.
If it is legible I don't care. If I can select a style a little bit more specific than "serif" or "sans-serif", that's great, as long as it degrades to something legible. I've yet to see a situation where free-flowing text or headings became illegible because the font changed. UI elements, sure.
It simply puts a bit more trust on the user than normal. And if your OS has a ridiculous system-UI font that can't handle paragraphs well, you have bigger problems than some website.
My resume has no need to look any particular way at all; this is just my preference. You should design your webpages however you like!