And how would you fix it?

Personally:

Heroes:
Tala- weak but playable. Just has a lot of bad matchups.
Raikka- weak and frankly there seems little reason to choose her over any other mage. With shadow allies she can't even rush well. When frostmare was -2 cost to cast was probably a lot more viable. OFC -2 cost frostmare would make rush zaladar more unbalanced. I think her ability needs some kind of further tweak perhaps also making enemy allies who take fire damage ablaze.
Skervox- Also seems on the week side of the spectrum, but playable. Against solo decks (including mage solo or mage rush) he is absolutely awful which further weakens him. His ability is fine, but there needs to be some item/ability to function as an SE sink against solo decks where your ability and the shadow fonts he tends to bring are completely useless.
Ythan- Underpowered at present, probably playable and I can't think of a tweak that wouldn't potentially send him to being overpowered.
Loest- a bit meh but certainly playable. Change his ability to "killed/destroyed and exiled". It doesn't fit the concept that he can destroy Aeon but not a lowly spirit warden.

Rothem + Loest- both are a bit lacking in power (assuming desperate tactics gets nerfed) albeit playable. As legendary heroes it would be very nice if they both had access to something else unique be it an item, ability or ally. Ideally this would be some kind of minor effect on their respective armours. E.g. Loest takes one less damage from Loests burden and Rothems armour has +1 durability when worn by Rothem. Or the armours cost one less resource to summon.

The rest of the heroes seem to be fairly decent.

Tribes:
Ravager tribe is in dire need of help still. The lack of a decent T2 ally in the tribe is a big issue. If you place an item T1 then if you use it for spitfire hound you won;t be able to use it for ravager zealot the following turn and ravager alone cannot kill any decent 3CC allies. Up spitfires attack to 2 or allow it some kind of resource acceleration (items cost one less to play) or have it turn into an item if it uses its ability (in a similar manner to an ally affected by baleful transmogrification).
Templar tribe- very good in preist decks. Definitely playable outside of them as well.
Undead- perfectly good trible.
Twilight tribe- almost good enough. Twilight Eidolon is the issue here. Personally I would change it to "if you have two twilight allies, draw a card- costs 1 durability. Total durability 3. Activated ability: 0 se- whena friendly twilight ally is killed it is returned to play and twilight eidlon is destroyed.

Individual cards:

Desperate tactics- desperately needs a nerf.
Ascetic of Aldmor- great if going second. A waste if going first so basically too situational to include in a deck at present. I'd sugest making him a 1/1 or 0/3
Zailen Crusader- overpowered. Great fun and opens up so many options that I wouldn't want to take away the attachment love. At the same time, I don't make a human deck without 4 of her. Change to a 2/4 ally or reduce cost of attachments by 1.
Growing Bog Dweller- I've given him a few tries but just not seeing the joy. I'd reduce the ability cost to 0 similar to Knight of Unaxio. Whereas knight can consume spare abilities you don't need, bog dweller either needs you to have extra resources to play fodder to be killed or to reduce your board presence. Sure occasionally you might have some crippled ally, but then you probably wlready have other options for dealing with that.
Infernus- never seen this dude survive to be of any use except once through a portal (in which case I'd still rather have scourge). I'd change him to 5/7
Plightbred runt- the effect just isn't worth including him andd he is a piss poor T1 drop. If he was a 1/2 then firesnake would be pointless. I'd make his discarding more useful such as +2 attack to ONE ally.
Fatebreak familiar- his passive is too much a two edged sword atm. In particular in many cases I would rather he suck up that crippling blow/ poisoned arrow/ consuming fear so I can recall him to hand or get a smack in if he survives the poison. Also these abilities can easily be brought back later by their heroes anyway. If his passive is changed to "that ability has no effect, is exiled and fatebreak familiar is killed". This minor change would mean he could then be a useful ally.
Thunderstone Golem: even in a nish deck with discarding, shock conductor and lightning strike he is insanely awkward to play and even then phoenix is probably still better. change to a 2/5 or 3/4 3CC ally. It would be really nice if Nish could be a more competitive mage when running allies and this could make that happen.
Evolving abomination- Also not quite good enough. Even in an aramia deck set up to boost allies he just lacks authority on the board compared to any other 3CC ally. From 4 resources on he often didn't get a boost at all. Lovely idea, needs a +1 boost to either attack or health though.

Generally I think most things in this set are fairly well balanced. There are some attachments and abilities I can't really see being used outside of meltdown but other than that only the mage and ravager tribe allies stand out as needing help.