Tanking

Even without knowing, this can be predicted almost exactly from the overall archetypal design, as Dygz says. A better way to put it would be 'what does the Primary Archetype do that does not contribute to tanking and therefore makes them suboptimal?'

Mages don't sustain tanking as well, certainly not without another Mage to give them extra mana for the job. Result - expensive, inconsistent (won't even matter if they are squishy or not)

Clerics don't generate enough threat without the same issue, takes too much energy, they do much more DoT and mitigation which are better spent on others
Result - Teammates have to hold back a lot, making everything slow, or they have the mage problem

Rangers lose damage when the enemy is too close to them, in the current intended implementation, and also lack sustain somewhat
Result - Often can be secondary tank only, or combine with another tank, or requires teammates to give up
damage for defense, but at least they can offset this with their own provided damage. Quite hard to do

Bards generally don't generate much threat either, and tend to spend more time not building melee DoT. Magical/performance DoT usually doesn't generate any.
Result - Relies on melee for damage but isn't necessarily even up to Tank DPS levels, no instant damage mitigation (they could be nice and allow you to flip 'generating hate/threat' on, by applying the Threat augment to a Performance, but even then it'd still be hard to control the enemy)

Summoner - Pigeonholes a class that is supposed to do multiple things, into one thing. In FFXI summoners don't tank because rotating your summons is optimal for buffs and adaptation, but you can't just 'rotate out' your big tanky summon and everything be fine, in any longer combat. Also summons are harder to heal.
Result - Even if you can heal summons or they get the healing from on-hit abilities or similar, it's a lot of work. I'd still expect this to be one of the more viable X/Tank

Rogue - Either loses a lot of their damage from positioning bonuses due to the enemy always being in their face, or has to build for high evasion rather than melee output, hampering them
Result - Why even try? Half your Rogue abilities probably don't work if you maintank so where is your fun? Tank/Rogue with Rogue-gear would do this job so much better (at least with the augment styles I expect). Probably still possible though, since you could gear tankily and hope that evasion balances out Tank's natural higher defense. It usually doesn't, and your healer works much harder unless your evasion is much higher (I say this as a Ninja tank from other games). This could still work, though, so I expect there will be Rogue/Tanks out there.

Fighter - Now we're getting somewhere, but Gap Closers and Strong Blows don't do as good a job on most things compared to Tank abilities, they're normally tuned down, damage wise, so initial hate generation is an issue.
Result - Probably the most viable, but with the only real advantage being DPS, so limited moreso to things that don't hit very hard, but die fast, for some reason, and killing them quickly is a benefit. I actually expect to see a lot of these in areas with that enemy type, for 'fastest EXP'!

If I were designing this, I would make minimal effort to alleviate any of these results. If you consider the result to be fine and 'a challenge', or 'works for you because of what you want to fight', great. If you want changes so that you don't have those problems while tanking " It Who Sunders The Land "...

We have not received permission from Dygz to share the quoted post.

Edit: Lacerate is an early, powerful skill... Rogue Augments may play off Bleed effects... The Tank/Rogue is called 'NightShield', with a likely shortening to NiS... which is as close as you can reasonably get to 'Nin' in Ashes' lore...