I can answer this since;
The steps a person must go through to validate their identity and get a gpg fingerprint are vigorous.
Uploading packages to a repository require that digital fingerprint and that you digitally sign each and every package.
During this process, you must pass through lintian.
Once done, the launchpad server installs your software on a Virtual Machine and tests it. Any failure can result in that package being rejected.
I have had simple or small errors cause this rejection and you must go in and see what happened... And I am not alone in this. Most any repository holder deals with this. I have managed to not fail any builds for a while by being tedious, but ZorinGroup has had a slightly less lucky time:

You can imagine if that if the conscientious and professional ZorinGroup can still get some Failed Builds (0.02%), anyone can.
It is due to this rigorousness that many devs push for Flatpak or Snaps as that is less vigorous and far easier to upload to.
I can assure you that using a repository is Quite Safe. As soon as you connect to the internet and communicate with other servers, there is always an element of risk. But when that risk is far, far far lower than a Shark Attack when you live far inland; it simply is not worth worrying about.


