Unlike Ork and Space Marine durability upgrades, Eldar upgrades apply only to their shields. This means that they generally work out to the same effective increase even if the upgrade has a higher percentage on it. For example:
Ork/SM infantry upgrade: 15/30/45% for Lvl 1/2/3, total 90% health increase.
Eldar infantry upgrade: 25/50/75% for Lvl 1/2/3, total 150% shield increase.
Eldar infantry are 60% shield, 40% health on average, so the effective total durability increase is 60%*150%=90%. There's no problem here.
The Eldar vehicle durability upgrade affects both Eldar skimmers and Wraithguard/blades. Like infantry, Wraith units are roughly 60% shield, 40% health but the skimmers are about 25% shield, 75% health. To see the problem:
Ork/SM vehicle upgrade: 15/15/15% for Lvl 1/2/3, total 45% health increase.
Eldar vehicle upgrade: 25/25/25% for Lvl 1/2/3, total 75% shield increase.
Since shields make up about 60% of Wraith unit durability, the effective increase is 60%*75%=45%. There's no problem here.
Skimmers, on the other hand, are 25% shield, 75% health so their effective increase is 25%*75%=18.75%. While Wraithguard/blades and SM/Ork vehicles receive a 45% survivability buff from these upgrades Eldar skimmers receive only 18.75%. This seems like a problem when all units gain 45% more damage from the damage upgrades.