Pete Hunt, one of the original developers of React at Facebook, began work on a Cassowary-like layout engine at one point and came to the conclusion (whether rightly or wrongly) that arbitrary constraint-based layout is too user hostile to pursue further.

I too like constraint-based layout, but I have to admit flexbox is nice in that it has very predictable behavior. The main hangup I have with flexbox is that it doesn't allow for layout based on aspect ratios, but Yoga has extensions that resolve this issue.