In another thread about this topic people were going over cases where this item could cause serious problems in how effective the item is against squishy targets. So I decided to do some further calculations to see how possible this passive was completely unfair. I decided to do all of this based on Jinx VS Janna(+Sightstone) and using Dominick's Regard's Giant Slayer passive since I might as well use the slightly better passive and it is the more realistic one to see in a level 18 environment. Jinx VS Janna is also a somewhat common occurrence and Janna is also universally recognized as a relatively squishy champion.
For all of you who do not know Lord Dominick's Regard's Giant Slayer passive is you do 1.5% more physical damage to a target for every 50 more max health they have than you up to 15%. Basically if your opponent has 300 more max health than you, you deal 9% more damage (7.5% for 250, 12% for 400, etc. up until 500 health).
In this example Jinx has 1912 health at level 18 while Janna has 1813 health at level 18 and 1963 health with only a sightstone. This results in an already 1.5% damage increase for Jinx against Janna.
This only increases when we account for potential runes or Janna just buying a Locket or any item with a health Stat on it since it will probably increase Jinx's damage by a percent close enough to her survivability that is is a wasted stat. Let's use the case with jinx/Janna+SS and buy a locket on Janna which puts her at 2363 health and no armor to complicate ehp against jinx (besides the shield). This increases her ehp against Jinx by about 20% but also increases Jinx damage to Janna by 12%.
This could be further complicated by crit since it does not say whether or not the damage boost is applied before or after a crit is determined (potentially multiplying it by 2 or 2.5) and thus potentially making this a bigger problem. If it IS multiplied by crit and say Jinx has 50% (Zeal/ER/Crit Cloak+any of the 30% items or IE+any of the 30% items) and if Janna were to be unable to die for the sake of infinite auto attacks to make the crit chance of 50% be an actual 50% occurrence, then the increased damage to Janna is effectively 17% or 21% with IE. This means that if Jinx has IE and 50% crit that if Janna buys an item with 400 health (such as locket) Jinx will actually do MORE damage to Janna's effective hp (as a percent) afterwards than beforehand which is total bullshit. In a 1v1 environment she is actually straight up punished for building that item since she gains nothing besides a shield/health regen/cdr out of it at this point.
It is edge cases such as these where my problem with this item start to occur. If you really want a tank bust item you either need to increase the health gap to a much larger number or make it extremely math-y and make it based off of how much health your opponent has compared to you as a percent. If your opponent has like double your health for instance you would do X% bonus damage but if they are only 125% of your health (which is what the current 500 is close enough to) you will do significantly less than the double. With its current iteration it is a 15% damage increase that is stopped the second you hit the 500 difference. That tank with 6,000 health is going to take just as much bonus damage as the midlaner next to them with only 2,400 or so and THAT is a huge problem on a "tank busting" item.