Rob Graham, @ErrataRob, captured an essential truth when he tweeted:
Wheel re-invention is inherent every new programming language, every new library, and no doubt, nearly every new program.
How much “wheel experience” every programmer has across the breath of software vulnerabilities?
Hard to imagine meaningful numbers on the “wheel experience” of programmers in general but vulnerability reports make it clear either “wheel experience” is lacking or the lesson didn’t stick. Your call.
Vulnerabilities may occur in any release so standard practice is to check every release, however small. Have your results independently verified by trusted others.
PS: For the details on systemd, see: Sergey Bratus and the systemd thread.