It's less of a problem on Firefox because you aren't forced into auto-updating them. But yeah, Stylish is the biggest example that comes to mind.
I prefer using Greasemonkey / Tampermonkey but the ecosystem is full of sketchy scripts too and some people foolishly have auto-updates enabled. Also it's bizarrely really hard to get someone to use such a user script if they don't already have the parent extension installed, but if you package it as an extension on its own they'll try it much more easily.
Same. I have 4 browsers, 2 of them loaded to the teeth and the other 2 untouched since installation, one of them the one that I use for "mandatory" stuff or that you really need it to work, like banking or gov sites.
I prefer using Greasemonkey / Tampermonkey but the ecosystem is full of sketchy scripts too and some people foolishly have auto-updates enabled. Also it's bizarrely really hard to get someone to use such a user script if they don't already have the parent extension installed, but if you package it as an extension on its own they'll try it much more easily.