Well if this was true "Every decent Linux distro has a package manager that covers 99% of the software you want to install" we wouldn't have to install it thru sh.
I have some confidence that at the least the Debian packages won't be changing rapidly, making it more likely that any problems will be discovered before they get to me. A script (or tarball) fetched and installed from the Internet can change literally from one second to the next. If a trusted site is compromised the next download could be tainted.
Not all "package managers" instill confidence. I've heard too many bad things about npm and the associated environment and won't have it on my systems, but I am not a web developer so the impact is the occasional utility I have to forgo.