I have a plex server running on my headless linux server, and I'm not exactly sure that I'd call it a resource hog. I use it to watch TV and movies that I've ripped from BluRay, as well as some downloaded shows. I'm incredibly happy with it. I run it on a machine with a web server, mongodb database, and obviously my sftp server. I never really notice massive performance hits on the machine.
Sync is a bit of a pain to deal with, but once you figure out how it works (it's a 3 step process and the client never tells you what step you're on) it's manageable. I usually queue 4 things to sync, let it run overnight, and in the morning I'm good to go. Online content (Daily Show, Conan, etc.) can sync, but sometimes it's not supported so that's a bit frustrating. Syncing 6 movies to your iPad for a long flight is a huge improvement over the memory cards I was using for my Vita before.
The big thing, which apparently they're finally working on, is that it doesn't support music playlists in the client. I've never seen such a thing. How do you have a music player without playlists? It's my biggest complaint about Plex. But as I said, they're working on it....slowly...should be here by 2020. I'm using the music player that came with my phone in the meantime.