I didn't really think that Yennefer lacked characterization; I'm not sure what that complaint is about. Lack of Triss stuff and Eredin monologues is simply how the game is written, and I don't think it's somehow broken. Ideally, it would have been cool to see more of Dandelion, Zoltan and Triss, since they have been there in all three games and are three of Geralt's closest friends, but I don't think it somehow spoils or flaws the game. There was obviously a conscious decision made to introduce more characters instead of giving more screen time to the returning characters, and while a lot of players are not thrilled with that, it is simply the choice that they made.
I'd agree though, that tossing out every decision the player made in the first two games was pretty crummy, and it's probably my biggest complaint about the game, other than the fact that I finished it and it's over. There are some things about Act 3 that genuinely seem rushed (Reason of State, the Tower end sequence, the Lodge), but strangely enough those don't seem to be the main complaints.
Yennefer
If you've never read any of the books, you won't really know who Yennefer is. If you've never played one of the previous games, you won't have the first clue who she is. So who she is is first introduced during the dream were she's (rightly within the context of the dream's time frame) Geralt's romantic partner. But what do we learn about her? That she likes Lilac and Gooseberries and likes to wear Black and Silver. We next see her at Vizima were she's introduced to the player as the mother figure for Ciri. But what do we actually learn about her as a 'person'? Precisely nothing. You only begin to get the characterisation of Yennefer once you reach Skellige. The problem with that is that you may have already romanced Triss and because of a poor decision to lock players in to the romance, you cannot change your mind on that playthrough without loading up prior to the events of
Now or Never.
Eredin
Eredin is the chief atagonist of the game but again, suffers from the identical problem as Yennefer, i.e. if you have never read the books, you won't know much about him and if you've not played the previous games, you won't know anything about him at all. So if you fit in to the category of not knowing who he is, the game's writing does a very poor job of developing him. Let's look at the character for a moment. He's meant to be Geralt's nemesis. Yet what effect does he ever have on Geralt? None, so lacking any effect, his interactions are not only limited but lack depth due to the lack of any meaningful dialogue.
He's meant to be the leader of a race of peoples who are looking to escape a dying world yet when we actually visit their world it's a vibrant and beautiful ecology so what's his motivation? That the world may end in some undefined time in the future? Geralt doesn't appear to fear him, Eredin doesn't seem able or concerned with pressing Geralt's buttons, he doesn't commit any real atrocities, beyond killing people from villages, which your average Scoia'tael band does so what makes Eredin the single biggest force for evil in the game's setting?
As a result of this combination of a lack of dialogue from, and a story that never scratches beneath his surface to show a nuanced villain, Eredin is reduced to being this shallow character the game
tells you is the greatest threat to the world yet never offers up demonstrable proof to back that assertion up.
Triss
Regardless of whether or not you romance her, Triss is an important part of Geralt's life and has been a very important part of the trilogy of games, being the only other character to have a major role in each. If you've not read the books or played the games, you will know nothing about her, save that Geralt had a romantic involvement with her in the past. Yet the way the game portrays it is as if it was a fling, something of no consequence. That's not how it's been portrayed in other games. Whether you want to romance her or not, the option is there to do so yet on what basis do you choose her if you have no prior experience of her? She's portrayed as Saint Triss who wants to save the mages and feels guilty about 'taking advantage' of Geralt. Yet the character is not only more nuanced than that but the writing never explores what exactly that exploitation was. For a new comer, there's no context supplied.
The OP talks about flow and this is something I've discussed in relation to Triss as a character in other threads. The flow of Triss is all wrong. She's introduced at the beginning of the Novigrad area, has three periods of interaction, two of which are optional and then is largely forgotten about again until Act 3 when the player is once more in Novigrad, where upon there's a short cut scene that provides opportunity to decide Geralt's future with Triss, who is then once more forgotten until the ending card comes up. So irrespective of whether you romance her or not, you have this very important character in Gerlat's life, who has hours of gameplay between interactions and ends up being reduced to a background NPC with a single word - "Well".
It's not that this game is broken, on the contrary, it's how it's meant to be, but just because something was 'written that way' does not detract from the fact that the game fails to build up important characters and explain important events. In some places the writing is some of the best I've read, in others it's some of the poorest. It doesn't ruin the game but it does make the game less enjoyable than it might otherwise have been.
It's these poor areas that need attention. Just because the ending is written to show Ciri suddenly going off to fight the White Frost, which until that point in the game, had been portrayed as an Ice Age or Magic used by The Wild Hunt, does not make it any less confusing as to why or how she's doing what she's about to do. Just because something 'is' does not mean that's how something should be. The writing in large parts of this game needs cleaning up with dialogue adding in order to provide much needed characterisation and explanations and context for certain events. Without it, the game will continue to suffer from a disjointed plot.