Sylas’ dominance is finished
Sylas was one of the champions that leading the S-tier list because in the hands of a right player he is furious and unstoppable, but at lower-Elo brackets, he was extremely difficult to play. The changes of his basic abilities will make him easier to play, that will positively affect his popularity in the low Elo leagues, but they will hit hard professional stage and Platinum and higher leagues. So no longer you will suffer from Sylas played by some professional Elo Boosting man.
His survivability against enemy pokes was reduced extremely, and now he cannot instantly clear minion waves. So his laning stage will be much harder, and that is why his rating fallen down from the S-tier to the B-tier as a Middle lane champion, and from S-tier to A-tier as a Top laner.
Yuumi is finding her place in meta
Yuumi’s situation is very similar to the Sylas‘ one. At the highest levels, these adjustments will be considered as a nerf, but the overall situation is not so bad. Most players are consisting of the low Elo players and for most of them, these changes will be quite a big buff. Like she has a bigger health pool, but now her dash is interruptable with any Crowd Control ability.
The second skill was changed in such a way, to be weaker if it is attached to the burst AP champions, but it will be used for ADC champions it will end with a slightly better outcome. So overall, her place is still in S-tier as a Support, if you mastered this champion well, but if your team (or you) are not good enough she could only pretend to the B-tier. But if you playing League of Legends duo, then Yuumi in the greatest choice for any champion. And if you will take a pair Yuumi and Aatrox, then literally nobody could do anything about this outstanding duet.