I'm not getting all the hate for Plex. Plex is amazing.
It's not just a media server. It's a full-blown server and media management and distribution system akin to running your own Netflix. I paid the one-time lifetime pass and it's been totally worth it. We stream to many Rokus, computers and Android devices amongst my household and immediate family.
Having paid for the PlexPass, all the clients I add to my Plex Home group are free so users don't have to pay for each client. I run it on a 6-disc FreeNAS RAIDZ2 system that has enough CPU horsepower to run Plex Server right on the NAS itself. The system is beautiful and amazingly capable. Just drop any old video file on, and Plex handles everything else: metadata, posters, trailers, organization, and any necessary transcoding to play any type of video file onto any client regardless of its capabilities.
I know Emby is popular because it's open-source, and perhaps some parts of it are better than Plex (I've never used Emby). But the reviews I've seen put them pretty close to each other, with often Plex having a slight edge overall. But probably ultimately depends on what you're doing with it and what features matter to you.
Anyone who flat out hates Plex probably doesn't understand how it's really supposed to be used and what needs it's meant to address. It's easy to hate the perfect screwdriver if you're trying to use it as a hammer.