Scheduled drills are relatively bad in my opinion. When I worked on cruise ships you always knew at least day in advance when the drill is going to be held. We even had meetings on the Bridge just before the drill to discuss where the fire is going to be, what fire parties are going to respond and how. A lot of crew didn't know where the fire is, but usually it didn't matter to them. Everything was completely staged and in my opionion, and a lot of other people, not very effective. The drill had to look good and it was just rehearsed like a stage play.
The consequence was that crew just get used to it and prepare for it in advance. They take their lifejackets to the muster stations, start gathering in front of fire lockers etc even without signals given. All of this is bad and was discouraged though.
We had a good opportunity when we were sailing for a few days without passengers to do a drill at sea during the night. I've suggested this to the Master, but he just shrugged and was worried that someone might fall over the ship and drown in panic as they never had a drill during the night (or at sea for that matter...).
But still, if you want to be prepared for an emergency, your drills have to reflect that emergency. So they are not announced and made as real as possible. People learn fast, don't underestimate them.