Steins;Gate 0 - Blemished But Beautiful

Without going overly into detail, the VN has two main routes, A and B, the split-off point is fairly early in the story. The true ending which the anime tries to adapt resides at the end of route A, however to unlock the true ending of route A you need to first complete route B, and send a D-mail back to the aforementioned split-off point. After you've done this, you then load a previous save that was before the cutoff point, and continue back into route A. It is easy to see why this would be an absolute mess to try and adapt linearly, especially in only 23 episodes - the director opted to only adapt route A and make some minor modifications to tie up loose ends that would have been explained in route B.

What complicates this even further is that some character arcs lean almost entirely on route B (the route that was not adapted). Most notably Kagari, my favourite character in the VN, got absolutely gutted in the anime. Her character arc was very strong in the visual novel, but it was contained almost entirely in the unadapted route B. Her amnesia was used to explore themes of individuality, looking to the future and becoming your own person; additionally her close physical resemblance to kurisu was actually explained and not framed as a weird coincidence. They could have chosen to leave her out of the anime adaptation entirely; instead they tried to fit a square peg into a round hole and it ended up feeling rushed and somewhat out of place. Something similar happened with the character arcs of Moeka, who had three-hour visual novel arc about learning to value oneself condensed into a single five minute scene in the anime, rife with 'fanservice'. Maho got hit too, to a lesser extent.

Paradoxically, the fact that the anime felt too slow at times could have been solved by making it longer (3-cour). The reason that episodes 4-15 with a few exeptions seem to drag, is that they don't really lead to a satisfying end result. 12 episodes is not enough time to flesh out the stories of three different characters (maho, moeka and kagari), especially when you have to work around the fact that these different stories are supposed to be taking place in entirely different timelines from one another. They opted to (understandably) focus mostly on Maho, but as a result, kagari and moeka's stories ended up half baked and it feels like time wasted.
In my opinion, Kagari should not have been adapted, there was never enough time. They should have used the time spent on Kagari to flesh out Maho even more, and the anime would have been better for it. There are several other characters that could have been left out entirely, most notably professor Reyes. A three cour season would have fixed all of this, but alas, 23 is what we got.