Why is Master Yi currently considered to be in a relatively weak tier?

As in title.

Doesn't seem to be that weak, can abuse Essence Reaver and Stormrazor and Guinsoo's.

In the attached link, he is tier C... in the same tier as Amumu and Reksai and Camille jg.

Sure, the meta is all about early-to-mid game, but he can snowball so easily even if he is a mid/late game champ. His mid game can be very strong as long as have decent income.