It is not a security problem if you know what you are doing and trusting the source. As a general way of installing software is is problematic because it is a risk when careless users execute untrusted code from the internet. Using this in cases where it could be seen a safe encourages such unsafe behavior and undermines efforts to train users not to do this. There is also the issue that websites are generally less safe than dedicated infrastructure of distributions. Those also typically ensure some level of quality control and auditing.
If I want to run a software written by someone, going to that someone's site and grabbing the source and/or binary straight from them seems like a pretty straight-forward decision, you don't need some middle man of a "distribution maintainer".
Besides, while I appreciate the efforts of the distro package maintainers, they are overworked and can't really give the amount of care this huge pile of software in the repository needs, not to mention that sometimes their efforts are counterproductive (IIRC Debian used to deliberately break some terminfo(5) records to work around problems in some other packages). And I definitely remember reading an article (though I can't for the love of me to find it) about a Linux distro doing an automated switch from some sort of RPM-like packaging to straight-up using Flatpak, with predictably horrible results of lots and lots of broken software.
