The game is surprisingly fun, probably my most enjoyed rpg-adventure game ever. Not like I played others though. The only problem that I have (and possibly others) is that there are skills that could really help in fighting certain bosses like Nyx & Nox in CH4. I know for a fact that N&N is an optional boss and can be skipped, but they're impossible without the right set of skills, which pretty much suck. I'll be waiting for that Unity conversion, I have my hopes up!
Actually Nyx can be really easy, what is truly hard is for a player to notice that she is different than most other enemies you find in the game, she is smart because
she will target your elemental weakness 90% of the time and once you notice this her fight can become trivial since you can just
use covenants that are "neutral" to all her 3 skills and avoid heavy damage by doing it, Mella is the optional one here you can just camp in her covenant and use her skill to your advantage while using Demonvolt and Groundswell (if you have it) to do decent damage by using them in the right moment. It's true that her fight can still be really hard for exemple for a player that starts a new game in chapter 4 because of all the optional content in chapter 3 that will be missed but for a player with the knowledge about her mechanics,
all 4 trinkets available, Nefris (better than Synophi/Khrys), Groundswell and the max level you can get she is not a challenge at all no matter the build you have.
About the "Unity conversion" know that her fight may just become harder since the objective is to replace the mechanics of previous chapters by the ones from the last chapter of this arc, if you want to have an ideia just replace one of chapter 7 bosses for Nyx and try to imagine how a fight with her would be.