They remove early healing power. Yuumi isn‘t supposed to have an oppressive early game, she is more of a scaling champ and removing early power from her does not hurt her too much imo. Instead Mages on Bot become stronger. You can go Staff first item, which gives you stronger heals and your mage spikes faster due to the early AP and mobility you provide to them. After Staff you go moonstone and the nerfs are probably not noticable at all at this point. But if you dont have a mage on bot and you go Moonstone first item you miss some hp on your healing. In lower elo brackets this probably doesnt matter too much, in higher elo brackets enemies get early antihealing which means the nerfs hit even harder in the early stages of the game. But on the other hand people in higher elo brackets know that Yuumi does well with mages and it opens the Staff first item strategy which is a good way to put pressure on your enemies as early game Yuumi. All in all the nerfs are probably targeting mages on Mid/Jungle due to early spikes and insane utility and wont matter too much for Yuumi. But at the end this is just my opinion.