I forgot about PPAs - I don't normally use Ubuntu-derivatives. PPAs are a little more dangerous if I recall correctly right? Firstly it should be an official PPA from the software developer, and secondly because it's a repo you have to make sure that it isn't going to eventually pull in packages that replace/break your system.

Safety for the ones I've listed:

• Flatpaks - Containerized, separate from system packages
• Cargo - Manual compilation, /home installation
• deb-get - One-off .deb from official source and doesn't try to pull any other dependencies in - worst case you fail the dependency check I think?
• Homebrew - Pre-compiled binaries or manual compilation if you choose, /home installation with local dependency network
• Nix Package Manager - Roughly the same as Homebrew, /home installation
• bin - Probably a single statically-linked executable, /home installation
• Docker - Containerized, separate from system
• Compile and install it yourself - Highest potential for things to go wrong as you're messing with system packages and probably working off of some developer's questionable compilation instructions (or even lack thereof).