I totally agree, Cinderhulk's % scaling isn't enough to make it a viable choice vs any other jungle item.
15% sounds like a lot, and it can be, but since it's only off BONUS HP not base or total hp it's not nearly enough.
Let's look at how Cinderhulk compares to other items:
**Cinderhulk: ** 400hp +15% bonus hp (460 effective hp). 10% lifesteal vs monsters (not champs), 150% mana regen in jungle, and 15 + (.06/level) aura.
Sunfire Cape: 500hp + 50 armor **(750 effective hp) **25 + level aura, larger range, and 150% damage to minions and monsters.
Deadman's Plate: 500hp + 50 armor (750 effective hp), increased movement speed and damage upon stacks
**Randuin's Omen: ** 500hp + 60 armor (800 effective hp), -10% damage from crits, passive attack speed slow, active movement speed slow.


Let's assume you have all of these items together in one build with boots and one other damage/tank item (common). That would be a total of 1900 bonus hp. That means that Cinderhulk's 15% would be providing an extra 285hp making it's total effective hp 685 (still less than every other tank item) and provides no armor or magic resist, and passives that have no real effect on objectives or in team fights (other than active smite). It would take a minimum of 2500 BONUS hp in order to bring Cinderhulk's tank effectiveness (effective hp) up to part with ANY of the other tank items.
Granted, it DOES make you tankier in the early game than going for devourer, warrior, or runic, but if you sub out Cinderhulk for a damage item, your early ganks will be stronger, and that frees up your #6 slot for a true tank item, making you do significantly more damage, and significantly more tanky than any build that includes cinderhulk in the late game.
In order to be considered a viable choice, or equivalent to any other tank item, Cinderhulk needs one of the following: 30-40 armor, or 30-40 magic resist, 100 extra base HP, or 20% scaling hp. Even with any of these changes, it wouldn't be as effective as any of the others unless in a full build situation where the scaling really kicks in.