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.

The big one for me is moving packages to snap. You can work around it, but that defeats the whole “works out of the box” aspect