Breaking Down Cinderhulk
So as everybody has figured out recently, Cinderhulk is basically the top pick jungle enchantment right now - so obnoxiously so that I've built it on AD Fizz top a few times and came out winning because weeeeee effort. Now, a lot of us aren't actively certain why Cinderhulk is such an oppressively scale-tipping enchantment, so let's break it down here:
Cinderhulk costs 1850 gold total. Whether you're in the jungle or not, you're going to reach 1850 gold a lot quicker than this item's cousin, Sunfire Cape, which costs about 1000 more gold. Sunfire cape comes with some better base stats thanks to that 45 armor it gives you (mm, delicious survivability), but here's where it gets tricky:
Cinderhulk starts becoming immediately more effective in damage at level 5 if you engage straight out of farming. As the passive ramps up to 10 stacks, you'll find the 31.5 damage that Cinderhulk deals max at this point outmatches Sunfire's own 30. Basically you're already missing out on damage if you don't take smite with you to lane (and considering that the soonest I've seen an active Cinderhulk come up is on a lvl 5 champ, there's no point where Sunfire is going to outright beat this enchantment). Obviously, the warm-up period means that engaging cold turkey is going to see you initially dealing less damage, but this is pretty negligible when it comes to the second part, the bonus health.
Cinderhulk packs up 350 base health bonus, plus 25% of your bonus health. This currently includes Cinderhulk's own health bonus, so right off the bat, this enchantment is throwing a total of 437.5 health your way - almost immediately matching Sunfire Cape. You can then add the mastery and rune bonuses you would likely be packing, and more often than not you'll find Cinderhulk outright overtaking Sunfire Cape's health bonus.
What sets these two apart initially, though, is the 45 armor bonus from Sunfire and the immediacy of Sunfire's passive. But let's weigh just sheer momentum in this - with Cinderhulk costing 1000 less than Sunfire, you're gaining this bonus far earlier than most champions would have access to the vast majority of their upgraded arsenal (the closest thing being Morel's 2300 cost). With over 450 bonus health so quickly into the game at such a low level, you're going to find you have a lot more bulk than meets the eye. While this health bonus may seem like nothing but bulk initially (I.E. no actual tankiness at first), the way that armor and magic resistance works starts to turn this into a very oppressive item.
See, there's a part of armor and magic resistance called "effective health", and what this means is it takes 1% more damage more of your max health to kill you for each point in one of these stats. So if you have 30 armor, physical attacks must deal 30% more of your maximum health. Maximum health. Not bonus or base - maximum. So that 450 health boost that you get is allowing you to eat up far more than what it says - if you're a tank and you're already packing 60 armor or magic resistance early (hello, Shyvana, you dick), you're gaining that 450 bonus health plus an additional 270 that can vary based upon armor and magic pen. You can trust you'll at least be packing 150 extra health off of that, though. While this doesn't seem like a lot, keep in mind that this is in addition to the damage reduction base you already receive from these two stats.
So add this sudden burst of bulk up with dealing more damage than the vast majority of champs could ever dream to put out at level 5 or 6 with cooldowns and unmaxed skills so far and you have a nightmare machine. In short, fuck you melee bruisers who didn't pack cinderhulk, your life just got several times more frustrating.
So, in no eloquent fashion, there's your explanation for why you're getting pubstomped by every fucker taking smite for this enchantment.