Heimerdinger's passive feels kinda useless under most scenarios
It's an invisible-power-no-gameplay aura, gives health regeneration (which doesn't help in a lot of scenarios), and in reality doesn't keep his turrets alive for any longer than if they weren't affected by the aura. If someone wants to kill one of Heimer's turrets, it'll die. Otherwise, the passive just doesn't contribute to any of Heimer's gameplay, other than making his early laning oppressive (and Riot tends to hate Champions who do this).
Meanwhile our newer Champions get all sorts of cool passives, but Heimer... yuck.
I could talk about Champions like Mundo, but let's focus on our inventor, okay?
health regen level 1 being 11.005 HP per 5 seconds compare top the second highest being
at 10.18 HP per 5 seconds. Now fast forward to level 18
at 26 HP a second.
.