IMPORTANT NOTE: If you have not read the last two Chronicles books, and do not want anything at all spoiled, do not read this. I despise spoilers, and would not want to ruin the experience for you.
It isn't that the final two books aren't well written. I'm pretty sure it is impossible for Anne to write badly (although I haven't read and don't plan to read her newest ones so they could be terrible for all I know). However, something changed with Memnoch, and the series slowly drifted apart from there. If you haven't read it, Memnoch tells the tale of Lestat being courted by the Devil himself and taken through the "true" history of creation, God, etc. Being the religion geek that I am, I liked this book a great deal. I took issue with some of the plot twists at the end, but all in all, I enjoyed it. After this you have The Vampire Armand, Pandora, Vittorio, Merrick, and Blood and Gold. Armand is a whiner, but it was a good read. Pandora and Vittorio are largely outside the realm of the Chronicles (with the exception of Pandora's involvement with Marius), but excellent books. Vittorio is beautiful and strange enough that I forgive its digression. Merrick is the first book in which Anne tried to meld the Vampires and the Mayfairs, but it works in this one. Merrick is distant enough from the Mayfair series that you don't get bogged down, and frankly I just loved the Voodoo in the story. Blood and Gold is oddly framed, but being that it is Marius, it's fine. Then the train wreck happens. The thread of the main Chronicles story gets lost somewhere in Armand, and since the next few books are digressions, it just drifts away entirely until she attempts to find it again in Blackwood Farm and Blood Canticle.
*Sigh* The problem with the last two books is that they really don't want to be part of the Chronicles. Were it not for the fact that Lestat frames the first and narrates the second, they would have no ties whatsoever and could be continuations of the Mayfair Witches (especially Canticle). Here's the issue: Anne broke with the Catholic Church years and years ago, and sometime just before Memnoch she began to reconcile with them, which intensely colored her writing. She began to to think of the Chronicles as "darkness" and even titled her fairly recent memoir Called Out of Darkness. She started the fictional accounts of Jesus' life (which on the whole are not terrible--again, religion geek) at this point, and even appeared on that horrific show The 700 Club (see my response to that HERE). Even more recently she has broken with the Church again, but kept her Christianity. But now I'm digressing a bit. The point is that this all affected her books immensely. If Memnoch wasn't enough to prove it to you, Lestat's philosophizing throughout Canticle should do it.
I think Anne felt some modicum of responsibility to her long time loyal fans to finish the series rather than drop it at Marius' tale, but rather than take the time to properly end it as it warranted, she decided to rush the job. Blackwood Farm seems to travel on an odd and twisty path that makes little sense at times, though it is, of course, beautifully written as always. Toward the end, we get another main character shoehorned into our little universe, and at first, she seems okay--one of those characters you can stomach but don't really grow to love. The first time I read this (and this time, because I didn't remember what happened) all I could hope was that Mona would fade out of the picture early into Canticle, though I knew she'd appear because of the way Anne structured the end of Blackwood Farm, ending it in the middle of Lestat making her a fledgling. How sadly wrong I was. In all the Anne Rice novels EVER, there has never been a major character that I didn't grow to love in some way--until Mona Mayfair. She is the most horrendous little monster, and it gets to the point that every time she opens her mouth you think you can't dislike her more. Then she proves you wrong. She effectively ruins most of Canticle for me.
Now, I have heard (a few) dissenting opinions. There are those that actually like Mona, though how I will never know, and I'm sure those people think Anne did a wonderful job closing out the Chronicles. I am not among them. She dispatches Merrick Mayfair into the great beyond with so little fanfare that it's insulting (both to the character and her readers) in order to tie up a major plot line in Farm. I mean, really, how in the hell did she justify that mess to herself? Merrick was a powerful enough witch and Voodoo priestess that she could have sent Goblin's soul into the light without flinging herself into the flames as well. Hell, she was aiming to do just that in Merrick with the ghost of Claudia. Just ridiculous. Then Canticle, instead of truly focusing on Lestat, as the last of the series should have done in my opinion, she centers it on Mona's involvement with the Taltos (from the Mayfair Witches trilogy) and her unending ability to be one of the biggest bitches in christendom (oh, and Lestat's inexplicable obsession with Rowan Mayfair). Most of a few chapters in the middle are her being bitchy, Lestat getting angry at her, and Quinn being the go between. It was unbearably tedious. Frankly, he should have let her die on her bower of flowers in Quinn's bed at the end of Farm and saved us all the trouble.
In the final analysis, what I think happened is that Anne felt she needed to give her fans an ending to the Chronicles and (for some unknown reason) one to the Witches as well, so she threw them in her mental blender and voila! She used Farm to shove Mona into the picture, giving the Mayfairs a reasonable claim on story space, and then she used Canticle to play out the end of the Taltos saga. I might not be so aggravated by all this if she didn't attempt to call them both Chronicle books. Had she simply been honest and presented them as a bizarre (and wholly unnecessary) extension of the Mayfairs, I would at least have known what I was walking into: A hot mess, but at least a properly categorized one.