Originally, I too believed that Yi was going to be stupidly overpowered with the release of sated devourer.
However, after a few games as and against him, I realized that he has only become slightly better.
For all those who claim that he is broken and theory crafts his damage, I ask if you have actually played against Yi today.
Yi before Sated Devour or would kill most squishies or tanks if he manages to get on top of them, with sated devourer, he just does it slightly faster.
However, he is still just as Vulnerable to CC and damage as before. Late game, it only takes me 3 AAs from an ADC to kill him. If I am with my teammates, him engaging on me is suicide. Yi can still be stopped the same way he is stopped before. He pretty much runs in a straight line most of the time to close the gap, so just aim your CC at him.
The only real benefits so far I see from his gameplay is that he can hit his power spike much earlier since Sated Devouror stacks faster, he can fight some tanky champions that he could not before, and that he can solo baron. However, these perks are not exclusive to Yi. Today, I also saw an Udyr and Aatrox solo baron by themselves. As well, they can also handle tanky opponents, and probably in a manner much better than Yi could.
Yi sure do a lot of damage; however, he is still manageable with the right abilities.
Xin Zhao on the other hand has a kit even more mindless than Yi's. Despite Yi's kit's simplicity, Yi must watch when he Alpha strikes to avoid CC or burst, when to Meditate to mitigate the most damage, and when to E and R since his damage falls off when they expire. Xin Zhao just has to decide if he can go in or not, and perhaps when to use his ult.
From season 1, anyone can tell you that Xin Zhao is much more oppressive than Yi if he ever gets ahead. He has built in regeneration and CC; it will literally be case of him killing someone before the knock up wore off.
Shyvana has been a lackluster top laner for a while; however, as a jungler she has also been sporting an absurdly high win rate. The key to Shyvana's success in the jungle is her ability to counter jungle and blatantly 1 v 1 most junglers throughout the game. Take Yi against Shyvana, and see how often you can win in a duel. Granted, Shyvana is easily kited so the nerfs may be unwarranted. I believe that Riot is afraid of further buffing her dueling power and counter jungle potential to make the fight even more one sided than before.