Black has two disadvantages starting though, second and additional time pressure. As for the win rates I can't imagine they'd be very good data because armageddon is the format of last result for tournaments so there's likely fewer games played. On top of that there's a lot of player variation under time pressure. Some (like Carlsen who has a rapid elo of 2880 and a blitz rating of 2939) have their ELO improve as you move to faster formats while others get worse (like, unfortunately for him, Caruana who goes from rapid 2789 to blitz 2767). The rating differences alone mean Carlsen is the heavy favorite to win going into these tie breakers well before we get to the armageddon round. [1]

I think given the probabilities in the 538 model Carlsen's decision makes a lot of sense. Why slug it out to a probable draw with the chance to lose out of nowhere when you can move to another format that drastically favors you?

[1] https://fivethirtyeight.com/features/carlsens-bizarre-decisi...