In my years of playing 5e (Since the beginning), I've seen Hypnotic Pattern completely shut down an encounter exactly once. Granted, the one time (when a group of Fire Giants went from deadly encounter to an easy mop up) was pretty great, but still - exactly once.If you have 10 enemies and hit 6 with hypnotic pattern then as long as the enemy first in initiative is not effected by hypnotic pattern then every enemy gets an action. 6 are used to wake up allies. 4 are used as normal. The spell is now over.
It's one of those spells that in white room analysis gets called broken but IMO doesn't actually play out that way.