Joe:
Stretch? Systemd was default init for Jessie, the previous stable. Worse, an upgrade of Wheezy to Jessie would actually change the init system used, thus breaking almost every Debian server in the world.
Nicolas George:
I must be lucky, none of the servers that I handle broke because of that.
You are. Every single Debian 7 system with systemd that I upgraded to Debian 8 hit Debian Bug #774153, meaning that the upgrades did not complete unattended.