The people saying CSS is unfit for UI have probably never actually built a good UI in anything, and also realistically don't even know how to use CSS.

Flexbox on its own is a simple and very capable algorithm that you can use anywhere. You can do a lot with flexbox alone (see Yoga: www.yogalayout.dev) so it's a great place to start. Taffy supports a few more layouts (block, grid) which have their own separate use cases but flexbox is enough for most things.