What made Yi/Shyvana super obnoxious was mostly Sated Devourer.

In the Feral Flare meta, Yi was a mediocre 49% win rate champ and Shyvana was the worst jungler in the game with a 46% win rate, behind even Urgot jungle who had a 47% win rate. This was despite Feral Flare stacking infinitely.

After introduction of Devourer, even though it was infinitely stacking, Yi stayed a mediocre 49% win rate champ while Shyvana fluctuated between 46% and 50% winrate.

After introduction of Sated Devourer and removal of infinite stacking, Yi leaped up to a 50% win rate, while Shyvana leaped to a 52%.

When Fervor of Battle and Strength of Ages were introduced, Yi leaped to a 52% win rate and Shyvana leaped to 54% win rate.

Seems to me that Phantom Hit is so OP, it is more broken than infinite stacking.

In practice, Yi and Shyvana were much like Nasus with worse stacking abilities: they don't actually get to stack all that much damage since they don't have time to farm mid-late game and they rarely get to just farm champs without outright winning the game. In my 300+ Master Yi games in season 4 and season 5, I never got more than 80 stacks on a Flare/Devourer, with the average being 50-60.

Note that the damage on Sated Devourer alone is equivalent to a 50 stack old devourer (90 average magic damage) which would take the whole game to farm.

Let me compare an 120 stack old devourer (where you are absolutely stomping) to Sated Devourer with Fervor of Battle:

Old devourer with 120 stacks: 150 magic damage per auto. Note that you can only get 120 stacks of devourer if you clear your entire jungle instantly when it respawns every single minute for half an hour. In reality you'll never get to do this since it takes time to walk from camp to camp, you have to recall at least once to buy devourer and you will likely have to fight sometimes.

Sated devourer + Fervor of Battle: 90 magic damage average per auto (60+120)/2 + 167.88 physical damage per second (111.92+223.84)/2.

It wasn't even their kit that made them super obnoxious, which is why the kit nerfs were completely confusing to me. It was just phantom hit on Sated Devourer, then combined with Fervor of Battle.