Part of the problem with singers was undoubtedly a problem with hearing onstage. Those shows are always hard to get monitors - even in-ears - just right. There is the audience roar, the big venue reverb and boominess, etc. I actually thought the sound was mixed better for the audience (including TV watchers) than in past years.
But there was some pushing; some trying too hard and probably some "nerves" that put "ceilings" on voices that don't usually experience problems with their high notes. This manifested in thin, flat and even broken notes.
It is SO HARD to be exposed so publically and fall short of the vocal mark, so we need to give them some grace. And some of you may not like a performance just because you don't like that particular style. But for the sake of learning what helps our voices, I want to throw a three part question out for you that focuses on the voice itself:
What did you see that worked (vocally) on the show? What didn't? Why do you think so?