In general it's the exciting high-churn buggier distros that switched, not the distros that people use for serious work, although eventually RHEL will probably switch.
This could more or less equivalently be described as it being the very conservative, release less than once in a blue moon distributions that haven't switched yet. In those instances, it's not at all clear that they're holding back on systemd for technical reasons. RHEL hasn't made a major release since before systemd was an option, and they're planning on switching for their next release. I think SuSE is in more or less the same boat. Debian stable is having a disagreement based on systemd's cross-kernel compatibility, not its technical suitability. The only big distribution that is clearly rejecting systemd is Ubuntu, and that appears to be more of a NIH syndrome than a technical decision.
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