Are there any risks or disadvantages to building software from source, compared to installing a package? Can it mess with my system in any way?
I usually avoid it because I’ve found it to be a faff and often doesn’t work anyway but in a couple of cases it has been necessary.
The main disadvantage is that it’s less automated, and also you don’t get automatic updates without any other package management system in place. If you’re using something like e.g. source packages from the AUR then that solves both those problems and there’s no downsides (beyond extra computational power/time you spend waiting) so long as the package maintainer does their job correctly.
Not… really? I guess if you’re downloading random tarballs off the internet and running make install without checking the integrity or trustworthiness of what you’re downloading then you could get a virus. But if you’re certain the source you’re getting is legitimate, then I suppose the only way building from source could “mess up your system” is if you mess up your system libraries or something whilst trying to install dependencies.