Urriak Uruk
Gaming is fun, and fun is for everyone
True... if you run them directly as that statblock says. Remember, the book says they have statblocks like spies or like nobles, not that they use that statblock exactly--and even then, you can change them.
So it's perfectly reasonable to say that Darklords can't be charmed or put to sleep magically or stuff like that. Or attempts to charm a Darklord bounce back on the caster, or cause psychic damage as the PCs open their mind to the alien evils of the Dark Power.
But even if you decide that no, Ivana has the spy statblock, no changes... that's OK. The PCs put her to sleep and slit her throat or get information out of her... well, they've now marked themselves as a target for every single one of her minions (who might be instantly aware that she's been affected in such a manner). And she's immortal, so she could come back the very next day. (And in Ivana's case, this would be a good way to get the attention of Ivan, who might just welcome the PCs into his playhouse.)
OK, sure, it would have been very nice if there was a section that read "Darklords can't be charmed or put to sleep magically, and are immune to the frightened condition unless it's directly related to their Torments."
But the lack of high power for the Darklords not new to this edition. Ivana was a 0th-level Human with 8 hp in 2e with no particular immunities, not even to poison (although she could always detect poison anyway, so it basically impossible to poison her). In 3e, she finally was immune to poison, and she was fairly high level: Aristocrat 3, Expert 4, and book-specific prestige class Court Poisoner 5, but even though she had a lot more hit points (115 hp), she would still be killed almost instantly in battle since relies entirely on close-ranged attacks (one of which is, admittedly, quite powerful) and doesn't wear armor.
Honestly, the only weirdness I find in the statblocks is that (A) they didn't say "If you own VGM, you can use this statblock from that book instead" Because let's face it, Vladeska should've been a warlord. And (B) in 2e, Hazlik was a 12th-level wizard but uses the Archmage statblock, while Tsien Cheng was a 20th-level wizard but only uses the Mage statblock.
I very much agree with this analysis. I think part of the reason it is set up this way, is that the designers thought of trying to structure these Domains as suggestions for different blocks of levels, to make it easier to structure a whole campaign by jumping from Domain to Domain. It's very easy to imagine a campaign starting in Borca, moving to Dimentlieu, then I'Cath, Kartakass, and ending in Har'Akir with the power level of Dark Lords (and their minions) scaling up as the players level, to keep the fear factor up.
Now, I don't think that's the only or even best way to run a campaign, and one could definitely run a mid or even high-level group in Borca, with Ivana being untouchable solely due to her web of connections and military protection. I do find the choice of statblocks indicative of something however, as you say it's interesting that they make Ivana a spy, when they could make her the far more powerful Assassin statblock instead (which would still fit her character well).