I've been a Plex-to-Jellyfin convert for a long while now, and I've nothing bad to say. I switched because Plex kept pushing their streaming services, and the fuzzy feeling I get from running FOSS. Jellyfin used to run on my desktop which has a beefy CPU/GPU, but runs fine on my puny Synology NAS.
I check out Jellyfin periodically, but the last time I checked it isn't quite ready to replace Plex for me. All the apps (ios, Android, Android TV) aren't great, crash, and I don't think the codec support or audio passthrough support was that good. Hopefully in the future.