I subscribed to AppleCare One today. I was able to add my Apple Watch and Mac, but receive an error with my iPhone 16 Plus and iPad Pro (M4). Chat Support couldn't help me so I set up a call for later in the day. Usually you can get a call pretty quickly; that the soonest slot was many hours from now makes me think a lot of people are having AppleCare issues right now lol.
If they don't let me include my iPhone and iPad, it's a disaster. I'll be paying $10 a month each for my Watch and Mac, which is way more than I was paying before!
My Watch and Mac were already on monthly plans, but my iPhone and iPad are on the two-year plans.
I assume they're just having system problems because there were a lot of people trying to get the service.