AP Yi is extremely strong against large numbers of squishies, low cc, low sustain, and lack of coordination, and also fairs very well against dumbasses without MR runes.
In other words, hes great against the large number of low-skill players that can be found in pretty much every aram game in existance.
However, he falls off against tankier teams, medium to high cc, and extremely high burst champions who can take him out before he has the chance to activate Meditate (Talon, Lebonk, Veigar).
Even just getting a Bulwark can help tremendously against him, and before you give me that "but im a carry i dont buy bulwark qq", I've bought Bulwark on things like Xerath and Nidalee before because my team needed the MR badly.
We ended up winning both games because said decision to purchase Bulwark ended up making my squishy-ass teams resilient enough to withstand the enemy poke to the point where we had just enough health to win the teamfights.
With that said, if the Yi decided to go AD, feel free to disregard everything I said and run around in terror.
The Dunkmaster knows no fear!