Liandry's Torment has an identity crisis

Is it a tank buster? % health damage suggests so. But the damage is underwhelming at best.

Is it a squishy squasher? 15 magic pen suggests so. However, the mpen is almost useless vs tanks. Consider this - 15 magic pen increases your damage to targets based on their mr as follows:

Target w/ 15 mr: 15% damage increase Target w/ 50 mr: 11.1% damage increase Target w/ 100 mr: 8.1% damage increase Target w/ 200 mr: 5.2% damage increase Target w/ 300 mr: 3.9% damage increase

This means that with 15 mpen you do the above percentages MORE damage to the target than you did without the 15 mpen. Since you aren't doing much damage vs high mr targets to begin with, the percent increase means even less. Below you see the absolute percent damage increase (target's percent damage reduction WITH your 15 mpen minus target's percent damage reduction WITHOUT your 15 mpen).

Target w/ 15 mr: 13% Target w/ 50 mr: 7.4% Target w/ 100 mr: 4% Target w/ 200 mr: 1.7% Target w/ 300 mr: 1%

Is it a tank item? The 300 hp suggests so. But it feels so out of place. Especially since there are many other items you can get hp from.

So in what situation should you buy liandry's? It feels that in every situation you are losing out on efficiency with this item.