Ubuntu moved away from Upstart with the release of version 15.04 (Vivid Vervet) in favor of migrating to systemd.

[2] The traditional init process was originally only responsible for bringing the computer into a normal running state after power-on, or gracefully shutting down services prior to shutdown.

This leaves it unable to handle various non-startup-tasks on a modern desktop computer elegantly, including: Upstart's event-driven model allows it to respond to events asynchronously as they are generated.

In this way it differs from most other init replacements (beside systemd and OpenRC), which usually assume and require complete transition to run properly, and do not support a mixed environment of traditional and new startup methods.

Since December 2018 the project website says that Upstart is in maintenance mode only, and recommends other init systems, such as systemd.