Can you get in trouble for playing on a Private servers

While you're not likely to get into any real legal trouble for playing on a private server, there's always a chance that the software included in your WoW launcher detects that you are doing it, which is against the Terms of Service. If this happens, Blizzard could ban your account at any time, and you'd lose it as well as the characters you had on it.

So whether this is worth the risk is up to you.