40% mPen + 33 mPen, even on a tanky boi (lets use Pyke with his absurd MR of 57.5, so 57.5 - 40% = 34.5) will mean you're doing true damage to every champion in the game except for Talon, Pyke and Sylas if they're to build no MR at all. Talon is left with 3.18 magic resist, Pyke with 1.5, and Sylas with 1.44. It's especially potent mixed with Liandry+Rylai or Ludens (strange how that works...)
Oh, and Sudden Impact makes sure not even Talon, Pyke or Sylas are safe from true magic damage™. It becomes 40% + 40. It's why champions able to easily proc it are so frustrating to fight: they will, literally, nuke you before you can react unless you build one or two defensive items, and that's the source of peoples' bias when they say "defense DOES matter!" It's just because, after buying a negatron cloak, it now takes two whole rotations to kill you so you can nuke them before they nuke you.
How Riot had been screwing with magic penetration has had major backflow, and some people are too stupid to see why they think tank items matter when in the grand scheme of things, two single pieces of cloth armor will do the exact same. Tank items are bad. The way they've been fucking with armor penetration (lethality, etc.) and magic penetration is making it worse and creating a divide people are too stupid to bridge the gap between, and I'm sitting here laughing at them as the game suffers because of it.
Literally, tank items are so bad that people don't even bother to finish them because it has too little impact. They sit on a cloth armor all game because anything more has no noticeable impact on how much damage they can take in this burst-heavy meta.