[noncovalence]

In addition to the advantage of explicitly specified capabilities mentioned by others, I'd say a userscript is also much more durable and low-maintenance than a Chrome extension. This is coming from someone who's written an extension for personal use before, and had to jump through a million hoops with Chrome Dev just to give the browser permission to run my own code that I wrote myself, and that's to say nothing of the whole mess with manifest v3 as well. Throughout the entire time I've been able to happily keep using userscripts written by myself or others with 0 maintenance required.