Right now, snap (2016-present). Before that, Unity instead of GNOME (2011-2017), Mir instead of Wayland (2006-2015), Upstart instead of systemd (2013-2017).

They always do something custom-made and not adopted by anyone else, only to completely backpedal and go with what everyone else has already been doing. So, even if you like their custom-made solution you'll eventually end up being disappointed. After that, it becomes like a relic that only some frustrated sysadmins like me will have to deal with whenever we interact with some legacy systems, which definitely doesn't help with Ubuntu's overall reputation.