The difference between Levi & Mej or Soto is that all 3 items encourage passive play, but the one getting Levi is either Vlad or a tank. In the latter case it's hard to keep the stacks; in the former case it's borderline OP.
I think all of those items would be better if the max stacks didn't wait 'til MAX stacks to fully implement. For example, Mej could grant +0.5% CDR per stack; 20 stacks gives 10% CDR, but you don't lose it all if you die once; Soto could grant 0.5% MS for 10% total (or maybe an AS buff instead would be preferred since nobody gets Soto anymore - BT being so much better and all).
In that line Levi could grant 0.5% damage reduction per stack, for 10% total; sure it's harder to get and keep stacks, but probably easier to balance around. Alternatively they could've given it a 1 bonus AR & MR per stack, for 20 AR & MR total.
Then again, people get these items for pubstomping, and pretty much only for that, so maybe Riot wants to leave them in for these niche situations. However, DFG >> Mej, and BT >> than Soto, filling essentially the same spots, so maybe these two items are also slated for expiration. With the new DFG, I can't imagine ever getting Mej again; I can't think of a single burst caster or AP carry that won't want it over Mej.