How strange! I had the exact opposite experience. By mistake I read Scion first. I then went in order, got to the end, and looped around to Dart. I thought Scion stood on its own extremely well and have been recommending friends start with it; I was almost unable to finish Dart and dismissed it as "Guess she hadn't learned how to write yet".

(My friend who started with Scion on my advice also ended up enjoying them all except Dart, and I suggested to Alicorn that she not start with Dart, she refused my suggestion, and then she disliked Dart and didn't read the others which was my prediction all along.)

Kushiel's Avatar continues to be my favorite of all of them, and the new Naamah series isn't bad either.