As I understand it:
1) Systemd has a much slower shutdown, which means a reboot takes much longer.
2) Debian can be rebooted in 30 seconds. So even if boot was speeded up, it is a negligible advantage, and hardly justifies such a radical change.
3) Linux servers are not rebooted very often, making supposed advantage even more negligible.
4) If a service is critical, there should be a parallel server running it. Which make boot time even less meaningful.
5) According to this article at Distro Watch: http://distrowatch.com/weekly.php?issue=20141027#qa : boot times are not improved. That has been my experience as well.