But what does Debian see as the risks of patching the software they distribute and how do they mitigate them?
Then there's stuff like: "this project only compiles with an obsolete version of gcc" so the alternatives are dropping it or fixing it. Closely related are bugs that only manifest on certain architectures, because the developers only use amd64 and never compile or run it on anything else, so they make incorrect assumptions.
Then there's python that drops standard library modules every release, breaking stuff. So they get packaged separately outside of python.
There's also cherry picking of bugfixes from projects that haven't released yet.
Is there any reason you think debian developers are genetically more prone to making mistakes than anyone else? Considering that debian has an intensive test setup that most projects don't even have.