Bristleback is a massive chunk. Bristleback is one of the most powerfully statted heroes, and snowballs from hero kills via his passive Barroom Brawler! He's like a one card combo. This is all kept in check by his rather lackluster signature card Viscous Nasal Goo.
Bristleback's 8 attack is the strongest of any Red Hero, and that's quite a feat. Axe and Ursa are the closets at 7, and Legion Commander can 'technically' reach 8 with her passive. His 12 health is in the higher end of red as well, equal to Beastmaster, but obviously lesser than the block of fat himself, Tidehunter. While he doesn't have armor by default, he very often WILL have some via his passive.
Barroom Brawler is the nectar that tempts us to run Bristle. +2 Armor on every hero killed in combat is a sweet bonus for something you already want to be doing anyway: Killing heroes.
Many people try EXCESSIVELY to get multiple procs on this. While, yeah, killing heroes sure is great, after a certain amount of procs this becomes 'useless' as he becomes 'practically invulnerable' at 4 or 6 armor.
But, I mean, if a passives downside is they become 'too tanky to realistically deal with and further benefit from', that's a pretty damn good sign.
Getting a few procs off this passive hopefully happens naturally. But of course, having spells like Duel or any other combat manipulating card surely helps the process. Even simple relocates like Blink Dagger or Rebel Decoy work too.
While piercing damage is an obvious counter to Barroom Brawler procs, there are many other ways to play around it. Creeps chump blocking don't give the bonus and just avoiding hero on hero combat altogether.
Some try to simply leave Bristleback alone. Cross-lane mobility is great to abuse this as after your opponent's tower blows up simply relocate him.
Many opponents hate giving BB extra armor and go extra lengths to prevent it. The mere presence of this passive can cause stress and friction in your opponents decision making - sometimes causing suboptimal play. Heh, I suppose that's another one of his 'passive skills'.
You know, a lot of people are really down on this card. I can see why but I really don't think it's as bad as people make it out to be.
First of all, the baseline use case of this is simply a 'cost to adding BB to your deck', and randomly giving units (preferably a Hero) with -2 Armor isn't the end of the world. -2 Armor is more crippling that people make it out to be.
People criticize this card for being overcosted, and IMO it's not. Combat Training is +2 Attack for 3 mana. -2 Armor for 4 suddenly sounds really fair. The downside to criticize this card for is its lack of impact. It does nothing against decks whose heroes you utterly crush already, and doesn't do much RIGHT NOW, requiring multiple instances of damage to feel the effects of -2 Armor.
If you want to maximize gains from this, you have to abuse mechanics that benefit from negative armor. Constant damage like Ignite or Plague Ward, as well as anything with Retaliate. While you shouldn't go through lengths to maximize Viscous Nasal Goo, it would be nice to have incidental synergies with it in your deck.
Which is why Legion Commander is such a potent partner with Bristleback. Duel helps you with his passive and her innate Retaliate decimates anything with negative armor.
Bristleback can definitely perform in any Red deck - but he isn't as OP as you imagined huh? He sometimes feels too one dimensional for many constructed strategies. I feel you HAVE to utilize the negative armor aspect of Viscous Nasal Goo to make Bristleback SHINE in constructed. I mean he can ALWAYS be good - he's simply too damn FAT! But to maximize his value, we can't just imagine his signature to be a wasted slot. For the most part, it's not bad to try out BB in your red decks and make some adjustment here and there. He definitely has the power level to merit experimentation.