Not only did the new font choice look problematic, but the samples they gave touting best practices often went for low-contrast looks. The Weather app displayed the weather on top of a lovely picture of a sky, complete with clouds. White text on partially white background? You are not making this easy for us, Apple. It didn't improve as I got down a little further to a section labelled Let color simplify the UI. Now, I'm a big fan of appropriate uses of color to signpost information BUT one has to consider that not everyone can perceive differences in colour. Obviously there are folks with colour blindness and I often like to use colour as a secondary marker for that reason alone. But then they actually suggest that the yellow highlight colour in Notes is a good example of this. I took a look at the screenshot and squinted. And squinted some more. I'm NOT colour blind. I can see them, but MAN, that's a recipe for eye strain right there, making those icons yellow on light grey.
That's just my mediocre eyes. There are folks with worse issues. An online acquaintance has a father with contrast loss, and she detailed his issues in this post where she explains that even navigating the unlock screen is now difficult for him. It's nigh impossible for him to get into his settings to change things around.
I think it might behoove Apple and other developers to preserve high-contrast skins of their OS and apps. Design isn't solely about making things look good, it's about retaining functionality and enhancing useability while doing so.