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.
Just convenience. That’s what packages provide. There’s no special magic under the hood in most cases as a downside to packages, and in most cases for specific projects, this is why stacks have containers, because you set the build steps to include the things you need in a pragmatic way, but now have to mess with static files on a filesystem.