Twice Dead King: Reign is a downgrade from the first book. Hopefully the ending will save it, but I'm sick of Oltyx sitting on a ship for 3/4 of the book and oscillating between moping and murder. Somehow I think he was less arrogant in the first book compared to now.
I was satisfied with the ending personally. The book overall was definitely a downgrade, but I felt the ending was fitting. It's very poetically tragic, kind of bittersweet. There is a scene towards the end
when they unleash the monoliths that I thought was very funny. I think my biggest issue with the whole book was that it felt like we were retreading his character development that I thought we had wrapped up in the first book. His arrogance only gets worse, and just when you think he's about to learn a lesson he doesn't. It kind of feels like there was too much story to comfortably fit into one book, but not enough to justify a second. I don't regret reading it, but I think you could skip it and not feel like you missed too much.
Do people hate returning primarchs?
Yes but no. There are people who hate them because they don't like them taking the spotlight of the setting. In theory having Guilliman running the Imperium should've eliminated much of the Grimdark setting as he goes about chopping heads and reinstating human rights. However I think the writers have done a good job really humanizing him as a character. Guilliman's Ultra-Depression is actually charming. In Dark Imperium you get to see why bringing back a Primarch doesn't automatically solve all of their issues. The nobels don't want to give up the power their families held for millennia. He realizes he can't tell a fundamentally theistic civilization to stop worshipping his dad because despite being space Jesus, they'll crucify him too.
The Lion's return was a very strange read. Where Guilliman's resurrection was the product of a long crusade across the galaxy and required the joint effort of Humanity and the Eldar, The Lion just sort of shows up. He literally wakes up in some sort of chaos realm, and then keeps walking until poof he's back in real space 10,000 years later. It is kind of nice to see how much The Lion and Guilliman contrast each other in their priorities. Guilliman, ever the statesman, immediately goes about trying to reform the Empire as a whole, reconquering the galaxy, and restructuring its armies. The Lion sees an immediate problem right in front of him, and feels obligated to resolve this issue first before he even worries about what the rest of the Imperium might be doing.
If they are going to continue raising primarchs one by one, by concern is why? Guilliman had a clear reason for his return, and we see the consequences of it to this day. The Lion came back and...well nothing really. He doesn't really want to change the galaxy, he just wants to do what he was made to do and wage war. He doesn't really serve a narrative purpose to be alive in 40k. He hasn't met Guilliman yet so we don't even have any good stories where he acts as a foil to Guilliman. It doesn't appear that he is setting up any sort of conflict with Guilliman either. The setting is much the same as it was without him. The only difference is now the Dark Angels don't have to chase after The Fallen anymore.
