I wrote a blog post about this which you can read here.
TL;DR: Entering special characters is a nuisance, even on macOS. To add an acute to a letter, you need Option+e; to add an umlaut you need Option+u. For œ, you somehow need Option+q. This is pretty hard to remember.
With ComposeKey, it's a lot easier. To get é, you simply type §'e. Ä is §"A and œ is §oe. Easy!
If you want to report an issue, go to the GitHub issues page.
You can contact the author on Twitter: @jqno.
Copyright © 2017 by Jan Ouwens
This program is free software, released under the MIT license License. For more information, see the License document.