Never thought the difference mastering writing can be so significant. Just like to add what I understand regarding this. It's rather about not making any mistake writing by hand ca. 1-2 DIN A4 pages while someone reads a text (slow enough). I can't remember exactly but making only one (or two) mistake(s) and it is not anymore excellent (just good). Making 4-7 mistakes and it is not good (just sufficient). Making few more and it is bad which means failed. It's a long text with a very short path to fail.
Ukrainian is less difficult to write. There are claims that standardization/reform of Russian made it more artificial (far from natural people language) with overtaking too many words from Latin languages. When I read / listen to Belorussian I think they have even more luck with matching pronunciation/writing than Ukrainian. Which suggests this language is even closer to the common roots old language. (I'm not a linguist.)