Wow CSS is unfriendly, you really realise how poor it is once you learn other layout systems (android's XML and iOS auto layout, etc). Wish there was another standard that could co-exist side by side on the web, something simple enough to be implemented reliably by the browser vendors.
I've heard that android's XML layout is terrible (from an android developer) and that iOS auto layout is borderline magic (from an iOS developer). Personally, I don't find css to be that bad at all, but I work in it at least a little everyday so it might just be stockholm syndrome.
It is inefficient. It is awkward to set up in code, so many people write their own syntactic sugar around it. It crashes hard when it isn't 100% sure what to do with its constraints. Working with multi-line text is still a pain, so you often run into clipped text on iOS if you dare to increase the font size. etc.