What are private servers like to a newcomer?

So classic wow is a lot more fun to me than retail but a lot of my friends wont play it cause of they sub. I was thinking we could play private server, something like wotlk because it seems overall the best in most regards. But there are a few things i was wondering.

Are private servers dead or well populated, are there active raid guild and pvp? Also are they the same as the wow of the time or are there major gameplay differences. Another thing ive heard is the servers are unstable and could be gone whenever. Also will i need a vpn?