Not a surprise, was there a compelling reason for upstart at all?
Not that I can think of. Upstart is slightly simpler than systemd to build configuration files for, but that's also because it doesn't have any advanced features like memory/processor/IO limits or the like. Additionally it doesn't even properly manage child processes... so you have to use wrappers to get Unicorn management working right in Upstart.
Upstart took years to add support for setuid/setgrp and still doesn't have basic features like a respawn delay which shipped in launchd (or 90s inetd) before upstart was released. There's still no integrated way to reliably kill malfunctioning child processes and the commands will block until success so you're going to need to kill -9 a lot when e.g. adjusting a script for a daemon which forks.
I was rooting for upstart for years but it's hard to take seriously as a project when basic sysadmin tasks are still gratuitously hard / unreliable.