To be honest: in my personal opinion, I think it is.
But since I've listened to Kalafina and FictionJunction the most, I'll just write my thoughts about those two projects.
My opinions aren't meant to offend anyone; that's just my perspective of the way things are.
I haven't been a long-time listener of Yuki like some people here are, but what first got me hooked on her music was Kalafina's sprinter. I really, really liked the seventh heaven album, because I loved so many things about it- the atmosphere, the vocals, the melodies, the way everything was put together. To me, the songs were beautiful and I've listened to them many times over these past years.
But, I never truly loved anything Kalafina released after that; Lacrimosa and storia I was okay with, but I couldn't listen to progressive more than a few times. I tolerated Hikari no Senritsu, partly because while it still contained Yuki's style, it was different and bright and didn't have so much of the same sounds. (For B-sides, lirica was interesting, a twist on the darkness Yuki usually mixes into her songs. The rest I didn't really care for.)
And then along came Red Moon, and I didn't like a single one of the songs in it (with the exception of bits and pieces of the title song). I felt like Yuki was out of her element, attempting to put her trademarks on songs with tired melodies that should have been infused with real sitars instead of electronic violins and Arabic instead of Japanese. To be truthful, I found the whole listening experience sort of painful, and I don't plan on listening to the album in its entirety for a good while.
After listening to the latest Kalafina single, I found it to be typical Yuki- typical to the point that it was boring. I love Yuki's style, and I wouldn't have started to listen to her if I didn't love it. However, listening to the same sort of style over and over, regardless of the quality, made me extremely tired of the music. A good composer should be able to successfully make positive changes to his or her music while maintaining his or her unique qualities over time, and I think Yuki just hasn't managed to make a successful transition thus far.
FictionJunction: I got hooked on FJ after being introduced to Kalafina, and I enjoyed Parallel Hearts because it showcased each of the four girls. Toki no Mukou Maboroshi no Sora, however, sounded like a darker version of Parallel Hearts, with the same formula in terms of which girl was used where and the arrangement of the song. I thought Everlasting Songs was decent, but it didn't consist of new material; it contained covers by Yuki's vocalists of songs that she had composed at an earlier point in time. As for the lives, I adored vol. 2 and enjoyed a lot of the songs that were sung. But I soon got bored of the string of lives that followed- particularly because Yuki decided once again that repetition was the best policy, and insisted on reusing the same songs in different performances (Mezame, zodiacal sign, salva nos, etc.)
Yuki's music takes up a considerable chunk of my playlist, but I'm deciding whether to stop following her releases and projects, because I haven't been enjoying any of them for a long time.