I'm at about Turn 75 in my Archaon campaign. (Normal/normal because despite watching a few hundred hours of LotW I don't have the stomach for harder difficulties.)
It's been a cake walk. The AI just doesn't seem as aggressive or predatory as THW2. Dealing with Throgg and Icetooth has still taken the better part of the campaign, that's not changed. But in general you have to do really, really stupid things to get a challenging fight. (I managed to get attacked by both Thrott and the Empire in one battle because I decided to siege a city where I was already looking at a valiant defeat. So they sallied out. Came out to 2 Skaven armies, the Skaven Garrison and an Empire army. Smoked them all. I imagine the survivors of the Empire won't be telling their kids about that particular battle.....)
AI armies don't seem particularly strong either in terms of composition. Still a lot of T1 units, and a sprinkling of T3 stuff. I haven't seen a T4 or T5 unit from anyone yet. I've had I think one settlement in the ass crack of Norsca taken but other than that no one has really tried to go for me, and I've made due with only three armies until very recently. I hit Strength Ranking 2 by around turn 40 or so, or at least that's when I noticed it. Usually in TWH2 by the mid game various factions would be looking to nip at the undefended parts of my empire because it's gotten bigger than my army count. Maybe it's the advantage of being back against the edge of the map and surrounded by vassalized factions as Archaon in TWH3. I've had exactly two battles so far where auto-resolve was calling it a valiant defeat, and I managed to turn them both into decisive victories (one of those was the above battle.) So not feeling particularly challenged by the early game, and the late game steamroll is well underway.
The game started with 272 factions and by turn 75 we're down to about 130. I'm really looking forward to the campaign end time-lapse of factions to see who went out when, and where.
On the topic of vassalage, shit be gettin crazy. The map is CRAWLING with armies from Norsca tribes now, and they're even making in roads into the Empire ahead of me while I clean up Norsca. I've vassalized 3 daemon factions and probably more than 7 Norsca tribes by now, with a few more to come. I even vassalized Clan Moulder, which, I didn't think Chaos could vassalize non-Chaos, Daemon or Norscan factions. So Now I've got Thrott running around fuckin shit up on my behalf. And yet somehow I was completely unable to vassalize or subjugate the Daemons of Chaos faction. No idea why, it didn't follow any of the other rules on vassalizing I've seen so far.
Other than that I've started to get a sense of vassalage now and it's sort of useful if a bit buggy. Some units seem to get stuck and will never respond to new orders or even move ever again. Some units are slow to respond to orders, particularly if they're already got previous orders. Maybe you need to well and truly need to cancel their existing orders first if you want an immediate change, otherwise it seems to take a few turns for them to actually do it themselves. The game mentioned something about AI obeying orders for a few turns before they'll go back to what they were doing. That only seems true of defend orders.
But yeah, vassals are fairly handy. They're good for camping on cities you think might be vulnerable, good for running down broken armies, good for ordering to follow you around to bolster your numbers. Just their presence alone makes the enemy AI have to reconsider things and might contribute to their seeming passivity.
That said it feels weird to not be responsible for every single campaign move. I'll look back at an area I've revealed but haven't moved toward yet, and found the AI has started capturing settlements there. I think it's cool thematically but it does make the game easier and like you're not really responsible for a bit of it. I still don't generally let them take Dark Fortresses and it is sometimes annoying to have to roll back a turn because they've decided to attack a target you were interested in, unbidden. I often have to order my vassals to go do something else to draw them away from my next conquest, because if you're looking to invade somewhere, often some of your vassals are thinking the same thing at the same time, since your enemies are their enemies. Economically I guess the benefit of vassals (for Chaos) is they can turn minor settlements (which make you no money if you own them) into small profit generators. Still not sure if that value outstrips what a fully upgraded Dark Fortress brings or not.
The Lord and Hero trait RNG as far as Chaos god allegiance goes seems a little wonky. If you're looking to do a mono god themed army in Warriors of Chaos, you're kind of reliant on both the Lord and Hero traits, AND which Paths to Glory they roll with. For example, I've yet to see a melee Lord roll with the Nurgle or Tzeentch traits that make it cheaper to dedicate to a god and get their Authority. But I have 8+ Lords in the recruitment queue each with the trait for dedicating to Khorne or Slaaneesh. And I've yet to see a melee Hero roll the dedication to Tzeentch Path to Glory. I don't think it's necessarily tied to anything, just RNG. My idea for this campaign was to run Khorne/Tzeentch leaning armies. But so far the faction I've had almost all my opportunities and successes with is Slaanesh. It's just a minor flavor thing at the end of the day, Chaos Undivided is going to be strongest in Archaon's army. Still, it's always annoying when a flavor thing you're trying to do in a video game is essentially tied to RNG. The game ends up over and trivialized by the time you get the things you were going for.
Lastly, one other thing I think is contributing to Archaon's campaign feeling a little too easy is the prevalence of Aspiring Champions available for warband recruitment. I'm not sure what governs it, but not long after you've taken over an area, you're as likely as not to see at least one Aspiring Champion available for recruit in any given region. You're more likely to see them than Chosen or Chaos Warriors, no joke. You're skipping pretty much the entire melee progression in Warriors of Chaos recruiting them via warband recruitment, and Aspiring Champions once your tech tree is built out are fuckin' disugstin'. Regen, frenzy, barriers, perfect vigor, strider, poison, immune to psychology....they make you kind of question the value and utility of mono god Chaos Warriors because they simply get so many (and many of the same) benefits of those troops. A Chosen of Khorne is just a way more vulnerable version of a decked out Aspiring Champion with way fewer perks. Kind of feels like mono god Chaos Warriors need another tier of troop above them, like Champions of <god>, so they can give Aspiring Champions a run for their money.