I'll be frank - Lady Of The Lake is perhaps the most unsatisfactory conclusion to a long lasting book series that I've ever read. 

If push came to shove, I could forgive a lot of the nuances that irritate me in storytelling; I can forgive "the multiverse", and time travel, and excessive flashbacks. What I cannot excuse is lazy writing; about 3/4 of the book could have been excised and not affect the outcome at all. Lady Of The Lake falls prey to distracted and disorganized storytelling and refuses to acknowledge itself as a conclusion to a series. Supporting characters, long under-utilized and ignored, conclude their story arcs by being killed off, as if the author did not know what else to do with them. 

I have accepted Geralt no longer being the main protagonist of The Witcher series, but in this book, neither is Ciri. Most of the POV is shared by old, inconsequential side characters or brand new characters that fail to leave an impact on the overarching story. The story lacks any sense of urgency, and we the reader feel unrewarded for the journey.

Perhaps the most egregious affront is the book's lack of finality.