Unregistered torrents (from upgrades to season packs or nuked releases) and the occasional upgrade paths that don’t always work.

My own upgrade paths tend to pull in some versions which get made redundant so every so often, just ensuring there’s no multiple copies as a result of said upgrades

You can use sonarr and radarr without indexers. There’s even built-in support for major trackers without need for additional apps like Prowlarr.

That said, I use autobrr to handle the monitoring of releases and let the other apps filter out what they actually want