I have a more favorable opinion of Demonology, but I also believe it has a few problems:
First the core aspects that I believe are problematic.
Doom: Even with the talent, Doom becomes a weaker version if compared to the BFA version, it does not generate a soul shard, it gets overwritten by Venthyr Convenant ability, it deals very little damage and now, if the Doomguard exist (I still haven’t seen proof of it) it can just kill your party members as it’s a not enslaved Doomguard.
Corruption: This is very much in the opposite direction of the current Demonology identity, it does not have any form of interaction with the rest of the kit and has a slow castime of 2 seconds, breaking the flow of Soul Shards/ Summoning of Demons.
06 Seconds to summon a demon: This is a huge problem and the class as a whole should push for this to not see the light of the day, it will kill M+, it will kill PvP and it will make leveling a terrible experience.
Then the Talents:
Power Siphon: I am a firm believer we should have talents that allow the Warlock to gain power by sacrificing demons, this spell is a good exemple of it but its weaker if compared to the passiveness of DCalling, I believe baking Shadow’s Byte effect on it would not only increase the power of the Warlock itself but allow some early burst.
From the Shadows: This spell follow the same idea of demons empowering the warlock, but its poorly implemented, affecting only 2 spells (Hand of Guldan and Demon Bolt) and not enough to be worth of use, this talent has no salvation in my opinion, it`s a boring talent, a weak talent and the slot should be used for something better (maybe something that sacrifices imps to empower the warlock too?)
Vile Fiend: I like this talent, it allows diversity in the types of demons you summon, sadly it’s a very weak demon, I believe it’s a demon balanced around being extended by demonic Tyrant, making the talent itself very weak outside of Tyrant windows.
My suggestions for it: Either increase the CD and power of it, or remove it from the Tyrant duration extending effect (similar to how Tyrants wont extend other tyrants) and lower the Vile find Cooldown.
Inner Demons: This talent is very interesting, but suffer from a poorly scalling of haste, allowing imps and random demons spawn to scale with haste would make it on par with Soul Conduit and Felguard.
Sacrificial Souls: This spell shouldn’t have been nerfed on BFA, and the damage bonus of it should be reverted, but I think it’s a important talent on the design of “Talents that empower the warlock x Talents that allow more demons x Talents that empower existing demons”.
Nether Portal: I don’t like this spell very much, but I know who love it and I know it deals a lot of damage (even on BFA the diference of it and DeCon is not all that high) I wish it could be easier of use, and punish less the warlock for having to move due to mechanics of for being interrupted.