The piece she was playing didn't seem that complicated but the added pressure of being the Duchess of Cambridge who is going to play the piano on national television must be quite heavy. So, very well done!
Absolutely, it wasn't in the same vein as Diana playing Rachmaninoff but it was still brave of her to do that in front of the World, especially as she isn't known as a pianist. It was very nice and I enjoyed the whole service immensely. Christmas is magical when it is done traditionally and the only two bits that spoiled it for me were Leona Lewis and Ellie Goulding singing in that dreadful drawn out fashion. I don't know one person who enjoys songs being strangled like that and yet many modern singers insist on doing it.
Watching her play (her hands and fingers), I don't think she's someone who plays Rachmaninoff or borrowing her former teacher's remark: "I don't think she was ever going to be a concert pianist.” (Or if we're talking about classical piece, maybe a simple variation of Pachelbel's Canon in D, but certainly not Debussy's L' Isle Joyeuss).
But there's also the fact that unlike Diana, Catherine played accompanying singer, not playing solo. Because it's different game. When you play in ensemble or accompanying someone, you're not only think about your play or the score, but also harmony. Not just following/reading the notes, but also reading/paying attention to others. It's tricky, particularly tempo.
Or like Tom Walker said
"So we got together, we rehearsed the song like nine times and by the end of it she'd absolutely nailed it, and then she went away for a couple of days and practised it, and then we finally got to do the recording of it.
"And I was really impressed because it's one thing playing along with me in a studio, just the two of us, but then to jump straight in to playing with a live string quartet and a pianist and two backing singers, all of which she'd never met before, and then doing live takes in front of the camera - that's a whole other jump from jamming."
(
Skynews)
So a lot kudos and hat off to Catherine!
Btw this article, dated 26 October 2021, talked in more detail about C's musical history; it's not new information, so it's surprising that her musical talents is a surprise.
Catherine, Duchess of Cambridge is a former pianist with a solid grade 5 music theory
(So when she took the baton in front of the Hamburg Philharmonic State Orchestra during Germany tour, it's possible that she actually knew what she's doing?
)
When a third party uploads something to YouTube, quality suffers. 
