I also help out at Mageia, the systemd packager (Colin Guthrie) is fairly busy but does find the time to add patches to systemd package (e.g. backporting fixes). Before systemd was integrated, he also fixed a fairly important journald bug. He doesn't work for Red Hat. Every time before systemd package is upgraded, he performs a lot of testing, often pushing things upstream again. It's not without bugs by far. Reason it is stable is continued coordinated effort across distributions. If you check the release notes of some versions there are fairly big changes every so often. It requires a bit of expertise to sort that out. Though a lazy distribution could just wait a little and backport the "stable" marked patches.

The problem is that systemd is taking a lot of work better spent in other parts of the project. Instead of testing systemd, it's better to user simpler components, systemd is evolving, introducing more bugs, and people like that packager have to make a Herculean effort instead of using other init system, even sysv! and improving other applications.