My biggest technical complaint about the show was that some of the female singers - Kacey Musgraves, Billie Eilish, Taylor Swift etc. - used the microphones on their iPhones rather than vocal mics, and as a result the piano drowned out their vocals. (Lady Gaga did it right.)
There were also times when the lack of effort was distracting. I love Paul McCartney, but all through his number I kept thinking "Why are we seeing so much of his ceiling? Couldn't he have turned his phone sideways?"
Best musicianship: Elton John. Second best: Keith Urban.