Well, I don't know if I share that opinion. I picked this up a couple weeks ago and to be honest, I think I like it quite a bit better than United in Regret. I agree it doesn't hold a candle to Celebration, but... that was a really unique album that would be difficult to top, especially with the frequency these guys have been putting out records. A Diamond for Disease was pretty damned great, but that's more just like a top-off to Celebration in my mind. After that they just seemed to want to put out more records. I thought Regret was decent enough, but I really only felt a big connection to the last two songs on the album, whereas Nightmare is more enjoyable for me in general. I don't really see any big standouts on the album, but it feels more competent to me.
I dunno'... I should really listen to it more. But still, I like it pretty well thus far. Definitely not Celebration 2, but still a solid entry.