Some more reflections from my current game with Argol. This is the army I'm playing:
https://magnusdurr.github.io/snapfire/w ... 8~411~22x1 (for some reason the "I'm Alpharius" thing doesn't play nice with Army forge save function.)
The "Coils of the Hydra" rule (semi-ATSKNF) works pretty well, making even almost eradicated small units stick around. A pretty fair compensation for not having daemons on the marines IMO.
The cultist units are quite versatile, but without daemonic beasts and with just heavy stubbers they have plenty of dice but problems hitting much. Overall they feel pretty balanced. Quite pleased with the way the lesser daemons work, giving some nice decision making and flexibility. Having a dedicated unit for the Daemonic Icon gives enemies something to snipe, and makes the formations more interesting. The Rapiers are a nice addition to a unit of cultists. On on hand, it makes them a lot slower (unless transport is bought which makes the formation gigantic). On the other hand, 3 AT4+ at 45cm is nothing to sneeze at.
The Light Transports are great value, but with the nerfs I've given them (less speed and worse armament) I think they're OK. It's possible to create some Orc-sized formations with them however, just like in L&D.
The Mutants have yet to see battle, but it's a unit that hard to win against without some serious effort. Great for capturing objectives.
The Hydra formation is incredible value, but also incredibly fragile. I think it fits really well.
The Rough Riders are a lot more durable with Chaos Hounds and Daemons, and the removal of the daemonic beasts prevented them from becoming overpowered.
The destroyers got stomped pretty early on, and they're super fragile with their 30cm range. Think they work really well. The headhunters' Sabotage caused a failed activation in the first turn, and haven't done much after that. But they're definitely extremely capable.
The Lightning Interceptors feels OK. Not much in the way of shooting, but competent interceptors. Might be too good with 1+ Initiative at 150 points.
The "I'm Alpharius!" rule has been tested to the max, with Argol having taken out 2 out of 3 Lords. I think this rule works really well, giving a very solid reason for taking Lords.
Last but not least, the SHT. Took 2 Fellblades, but positioned them quite poorly. After comparing them to what Black Legion can take, I think they're fairly OK. While 4 DC and Coils of the Hydra makes them hard to beat, they're not fearless like titans or Black Legion WE. This of course means that it's very possible to break them and then engage again, destroying them. So while they're certainly really durable and destructive, they can't go around on their own but needs to be supported.
To sum up, the army plays pretty much as intended, and so far I don't see any need to change anything!