I did not like reading this book, every moment was a chore.

This book probably had too much going on.
Maybe introducing lots of new concepts that seem to invalidate the rest of the series, making one question why they read this far. Twists that drastically change the course of what we signed up for.
Perhaps it spent too much time on the large cast of characters, that one could never got to be immersed in a single story, as the story would end too short, and we wouldn't care after the 6 chapters it took us to get back there.
It could be that Blood Mirror is afflicted with Middle Book Syndrome, as we know this is the last book in the series, so nothing major happens, just getting everything set up for the finale.

That being said, there were cool moments, twists that made me question what I thought I new in a good way. As well as some good character choices.

But I'm not sure it matters as at the end of the day, I just didn't care. If I hadn't already made the commitment to finish the whole series, I probably wouldn't have read the entire book.