The word on Octgn is the Female beastmaster is weak because her traits and abilities do not offer any great synergies.
It's more than just Octgn. Even a noob like me recognizes how hard it is to build a J-Beastmaster. Her 3 abilities and basic attack contradict each other's strategies. For instance, she is Fast, which is great for both melee and ranged casting (kiting) mages... but she gets +1 ranged for
non-spell (aka bow) attacks, which are all full action, which means she can't take a move action (to make use of Fast) on the same turns that she attacks with her bow. And while at first glance her Wounded Prey ability sounds like a built-in Marked For Death, it only works on non-Mage living creatures, which means it's only really useful in (specifically non-Necro) attrition matches where you plan to kill the opposing mage's creatures one by one. But this isn't compatible with her 9 Channeling and lack of summoning-related or channeling-related (or any other spell-casting or long-game) abilities. Thus, the J-Beastmaster is more suited to an aggressive rush strategy, and her Wounded Prey is largely wasted if she's racing to kill a mage. The next time I play her, I might ignore her ranged bonus and go melee - but then you question why not just go Straywood Beastmaster, who gets a +1 melee bonus. I'd consider a kiting attack spell strategy but she doesn't have cheap access to any attack spells, so they take up a ton of spell points.
She's just... bad.
Compare the Wizard, who has a built-in ranged attack that doesn't suffer from being associated with a damage type (e.g. Elemental Cloak doesn't help against it, aside from armor, etc). And he negates up to 3 damage per turn. And he gets cheap important spells. And he gets cheap attack spells of his choice. And he has 10 channeling + cheap access to the most channeling spells, and lots of spells to protect channeling upgrades and spawn points (including Wizard's Tower). And he has many ways to employ a very strong aggro (e.g. rush with attack spells to the face) or successful long-term attrition (e.g. mana denial, dispel/dissolve "undo" war) strategies.
It's easy to see why a Wizard is generally easier to win with than a J-Beastmaster. That doesn't mean a very good player couldn't win with the J-Beastmaster, nor that a new/bad player couldn't lose with a Wizard... but there are certainly clear disadvantages and advantages, respectively, of those mages compared to pretty much every other mage.
The Malakai Priest has similar drawbacks as the J-Beastmaster - e.g. a weak basic attack, low channeling, a damage-over-time ability that is very linear and he has to jump through hoops to get to work, a buddy ability that is highly restrictive (e.g. only works with a few creatures that aren't the best he has access to) and either requires your opponent to cooperate or you need to make heavy use of guards, etc. He certainly doesn't have the flexibility of the Westlock Priestess.
The Warlocks are kind of in the middle, as they have cheap access to two strong schools, and lots of mage-specific cards to help them with either aggro melee, aggro spell-casting, or long-term cursing and damage-over-time. They aren't quite as good as the Wizard, but you can customize them to whichever play style suits you and make them focused enough to be reasonably competitive.
The Warlords are a little trickier. In principle, they could be melee focused with a buddy or solo (especially the Anvil Throne Warlord), or could go for the long game with walled-in conjurations and slowly build up an army... but neither strategy seems to work as well as other mages in these areas (e.g. a Forcemaster generally makes for better solo; a Straywood Beastmaster or either Warlock tends to make for a better buddy build; a Druid is a far better conjuration/slow game mage; the Straywood Beastmaster and the Necromancer can each build armies faster and will tend to overwhelm the Warlord in medium/long games).
The Druid seems to be the queen of the slow game, with more tricks to slow down the opponent's assault and more ways to heal and turtle than even the Priestess. Since nearly all her creatures and conjurations have Regenerate, if she can control the board at all, she should be able to keep her side healthy while slowly winning the attrition war and/or damage over time - plus she has some tricks to speed things up, like Burst of Thorns. While she isn't as flexible and doesn't feel quite so "easy mode" as the Wizard, I feel when built and played right, she could be as powerful.
If I had to rank them based on my (limited) experience right now (e.g. after Forged In Fire, before Battlegrounds: Domination), I'd probably put them in this order:
1) Wizard
2) Druid
3-7) Straywood Beastmaster, Necromancer, Forcemaster, both Warlocks
8-10) Priestess, both Warlords
11-12) Malakai Priest, Johktari Beastmaster
