Overall, the Hakkari are, in their current state, a mixture of both Alpha Strike Offense and Attrition Grind Defense as a force. They excel in both, but much like a diamond, do possess a fractural issue.
In terms of Alpha Strike Offense, they can bleed themselves or prepared bodies of beasts/prisoners/etc. thus sacrifice blood to Hakkar to get short-term large boosts which can empower their bodies/strength/speed. Sort of the Hakkar-flavor version of the Bloodlust spell thing which the Bloodscalps mastered and you've worked on the tattoos for. But instead of drawing in strength from outside spirits and flowing them through the body to magnify muscles and blood vessels and what not, enchanting and empowering but then leaving you, the Hakkar one is sort of a blast of illegal steroids. The former is more sustainable and can be channeled more semi-regularly, supported with supplementary Loa aid and empowerment due to things being shared amongst the Greater Gurubashi Loa and Crethekk + minor healing spirits, the latter is not. Because Hakkar is a jealous bastard, and also just a jerk. But usually, the sheer level of Bloodlust/Bloodboosting in general from Hakkar lets them cause absolute carnage in the initial rounds, and if they spill enough blood doing so, it forms a good feedback loop of feeding Hakkar who will then channel a bit of that new strength into his slaves followers.
However.
If the Hakkari don't absolutely overwhelm in their first strike, and end up running into a comprehensive defense, it blunts them and reverses that feedback loop. Because Hakkar doesn't stop wanting blood just because the Hakkari aren't winning. If they're bleedin', he's feedin'. Which means that they start getting less gain and more drain from their chosen master. Goes badly in the long run.
Now, on the other time, Attrition Grind Defense, when they are the ones behind prepared defensive lines, they get to be the ones benefitting from the enemy coming at them and bleeding all over the place and empowering them without them having to bleed overmuch in return. This means that the better their defense, the more concentrated and hardened, the better, because that means that much more bleeding on the part of the enemy. Which, in turn, makes the Hakkari defenses grow ever stronger as they defeat the enemy attacks. Which means that in the long term, this can result in a practically invincible defense that is continually healing and growing stronger with each failed enemy attack. While also contributing to their Ultimate army ability: Summoning Hakkar.
And yet!
Ironically, the weakness of the Attrition Grind Defense is the Hakkari's main way of offense: Alpha Strike. If you can break through their prepared defenses with a strong enough force with enough power behind it, they literally can't 'spin up' the same level of slow but steady stacking empowerment. They start suffering from Hakkar Hunger attrition because they're bleeding all over the place as they try to defend. Yes, they will benefit from any enemy casualties, but if they can't build up stacks of empowerment without suffering their own attrition, they'll continually start rolling back. Historically, this is shown through how they were defeated the first time around. The Zandalari showed up with a magnitude of force and strength that let them batter their way through to Hakkar in the center of the city and blast them out.
At the moment, you know that the Hakkari are based across the northern Black Morass, and have concentrated their main defenses at their new city and temple of Atal'Hakkar, and at the western border of the Black Morass and Brightwood. You know that they are preparing to summon Hakkar once more, currently on a slow burn. You, as the Gurubashi, know that it is likely that if you push them massively hard enough, they might start rapidly self-cannibalizing their own troops in order to try and speed up the summoning of Hakkar, empowering remaining elites, etc. But at the same time, that will negatively affect them significantly as much as it will help in other sectors.
Is it possible for Hakkar and the Hakkari to win? Sure. And after he eats all the Gurubashi and drains the southern Eastern Kingdoms of life, an angry Aegwynn will then likely show up and wipe him from existence. And then the quest will be fully over.
But, as I've noted repeatedly in my DoDA thread, I don't throw 100% impossible scenarios at players. In which I mean with player decisions and choices, victory is possible. Rolls are rolls, but bonuses and success thresholds change with circumstances, which can be heavily influenced by player choices and things like that.
Will you lose? Maybe. Will you win? Maybe.
The maybe is the important part.