I have not read Sacred Ground, but am not surprised to hear it's being marketed as YA - as are quite a lot of books with fairly mature content. There's a Joan D. Vinge (Catspaw) that was originally marketed as YA, although the author never intended it. Probably a topic for another thread, but interesting and I think it speaks to our time...
As for Mercedes Lackey, she is definitely one of my favorite authors, though I agree that some of her books really fall short.
One Good Knight had some major disappointments (how fast did that romance get resolved?! Made my head spin!) but what I really liked about it was how it slowly dawned on the reader (at least this one) that the protagonist, Andie(?) was "supposed" to be evil. Her kingdom is a dark little evil one. I thought that was an interesting choice and nicely done.
I really like the Elemental Mage series - Serpent's Shadow and Phoenix and Ashes are my favorites. I'm glad to hear that there is a new one coming out (I never buy hardback, so I'll be waiting) It sounds more like the Red Shoes than Puss in Boots to me but I would expect Lackey to choose just one fairy tale for the Elemental Mage series - it's only the 500 Kingdom books that have multiple fairy tale influences (that I've seen)
As for the Obsidian Trilogy - Agreed that Kellen is one of the stupidest heroes of all time, I found his thick headed, short sighted, unimaginative person to be very annoying, and I think his sister would agree. Of course she is pretty stupid herself in a lot of ways (at least with regard to her romance) What I found very interesting and enjoyed (in a horror sort of way) was the depiction of the Endarkened. I think the demons are very well done. A good combination of truly horrific, but with a sort of evil beauty that makes their ability to seduce humanity still seemed convincing. Unfortunately it seems like that city might just deserve to get taken over.
I have not read any of the modern race car driving elf series, nor the Diana (Treegarden?) series. yet:)