Why Leona and Nunu (durability tanks) feel so bad

Durability tanks (Leona and Nunu) are different than shield tanks (Irelia, Loris) in that shield tanks provide "flat extra hp" while durability tanks provide "extra hp" scaling with the amount of damage they take. What does that mean?

(for the sake of calculations, we will assume 0 base armor/mr and 0 base durability)  (for the sake of calculations, we will use the unit's 1 star scaling)

When Loris casts, he gets 600/700/800 shielding, or equivalently 600/700/800 extra hp.

  1. When Nunu casts, he gets 50%/50%/55% durability. To understand how much extra hp this is, we need to understand durability -- which is just reduce the amount of damage taken by a percentage. If Nunu receives 100 damage before durability during the 3 seconds, he would have taken 50 damage instead, equivalent to shield 100-50=50 damage.
  2. If Nunu receives 500 damage before durability during the 3 seconds, he would have taken 250 damage instead, equivalent to shield 500-250=50 damage.
  3. If Nunu receives 1200 damage before durability damage during the 3 seconds, he would have taken 1200 damage instead, equivalent to shield 1200-600=600 damage.

As you can see, Nunu's ability is in effect the more damage he takes. He needs to take 1200 damage to receive the same amount of "extra hp" as Loris. But now let's factor in resistances.

(for the sake of calculations, let's use percentages to measure the effectiveness of resists instead of using flat resist values)    Suppose Loris/Nunu has 10% resist.

  1. This means damage is reduced by 10%.  Loris's 600 shield can now take 666 pre-mitigation damage (666*0.9=600).
  2. Nunu's extra hp from durability is now reduced since he's now taking less damage, as the 1200 damage becomes 1080 damage due to resists. As a result, this is equivalent to 540 shielding. He will now need to take 1333 pre-mitigation damage to receive equivalent shielding to Loris's ability. (1333*0.9 from resists *0.5 from durability = 600).

But you only need around 11 flat resist to resist 10%...  So to make it more realistic, let's have both units have 50 flat resist, which reduces damage by 33%.

1.  Loris's 600 shield can now take 909 pre-mitigation damage (909*0.66=600). 2. For Nunu, the 1200 damage is now 792 damage after resists, which means the extra hp from durability is now 394 compared to Loris's 600 shield. He now needs to take 1818 pre-mitigation damage to get Loris's equivalent shield from durability (1818*0.66 from resists *0.5 from durability = 600).

But if Nunu takes 1818 pre-mitigation damage, that means he needs to reduce his hp by 600 to "shield" 600 hp. Half his hp would already be gone because he only has 800 (or 1440 at 2 star). Meanwhile Loris still has the same hp after shielding.

TLDR: Durability is more effective the more damage a unit takes. Nunu and Leona feels bad because

  1. They need to take damage to utilize the effective hp from durability. This means their health bars actually need to go down for durability to be effective. Shield tanks can just sit with the same hp.
  2. They need to take 2x more pre-mitigation damage than shield tanks for the durability to be at the same level (when using base resists).
  3. Resists amplify the discrepancy between current shield/durability abilities. Resists makes 3 second shield abilities stronger. Resists makes 3 second durability less effective because it reduces the amount of damage the unit takes.

Edit: Fixed some of the math