I'd much rather be writing apps using constraint-based layout, much like AutoLayout. It took me a while to grok it, but it's much simpler once you get it.
I disagree, but I suppose this a matter of preference. For whatever it's worth, the React team did explore using a constraint-solver, but nixed the idea. They explain their reasoning here: https://www.youtube.com/watch?v=7rDsRXj9-cU&t=14m20s
Not sure what I think about React Native as a whole but I wholeheartedly approve of using flexbox for layout. It's much more declarative and easier to understand. Autolayout is a fundamentally flawed approach to layout. Building layouts in AL is very tricky and debugging them is twice as tricky as writing them. And lord help you if you have to programmatically add or remove views from a layout at runtime.