Problem with Ash warriors is that they're really is no standard for joining and they arent put through the same rigorous training, they learn the skills of being a bereserker and they're on their way with a Mabari companion.
You make it sound like they just tell you get angry then give you a dog...
Beserker's aren't just madmen flailing around with a sword. They're extremely talented warriors that focus their rage to grant them abilities beyond your common warrior stock (hence it being a specialization).
And the mabari warhounds would have to be highly trained to stick it's master, sent ahead to scout, track scents, disable foes.
All this training takes years of training and perfecting. You're dismissing it as if they just need to chug a few red bulls and run at people.
Remember that the Ash Warriors weren't just fighters, but skilled scouts that went into the wilds to scout for Cailin's army. They also survive Ostagar. That's no small feat.