On average I’d rate my experiences on private servers as poor. Some were better, some were worse. I only managed to land on a server with active, involved admins who actually put effort in once and that one was by far the best.

My favorite was one with insanely brutal settings and mods intended to increase difficulty. Never saw anyone make it past level 20 without rage quitting in several months there. Sadly it poofed one day with no warning like most of them tend to do. Though at that point it might as well have been my own private server.