Overuse of threads/coroutine's

Im having a bit of difficulty understanding how overusing threads affects my game. I fully understand that using them on the server is usually something to avoid, but what about the client? If I use a thread on the client every time someone attacks, would that be to much for my game/my clients to handle? Also, what all events create threads when their fired (EX: fireclients, fireallclients, fireserver, ect). Thanks in advanced!