I think we can say it is. writing-mode in particular has full support in all modern browsers, and some support in IE back to version 6: https://caniuse.com/#feat=css-writing-mode
In theory, yes, in practice (I speak from experience) it has a massive amount of bugs when you actually try to use it for something that is not completely trivial. And the bugs are different for every browser, so working around them is pretty much intractable.