That's a good point and makes a lot of sense. You can't just throw a daily down and expect it to be effective no matter what. Unfortunately, I think the DM had forgotten the part about only one Immediate Action per round and had the boss doing all kinds of other Immediate Interrupt tricks in the...
So I just had a 4e session last night where my DM tried to experiment with 5e Legendary Actions in a Solo monster encounter (the boss of a smuggling syndicate we've been hounding for awhile). One of the Legendary Actions he came up with was an Immediate Interrupt "Dodge" that triggered on a hit...