I'll start. Did you know you can run a headless version of JD2 on a raspberry pi? It's not the greatest thing in the world, but sometimes its nice to throw a bunch of links in there and go to sleep.
Radarr/Sonarr can't handle multiple versions of the same movie/episode, so a lot of people have a second instance for 4K movies (so they can have both a 1080p and a 4K version of the same movie). Also if you have a lot of anime it can be worth it to have a Sonarr instance configured just for that