I'm having some issues with this, I guess. What is the difference between YA and NA. Is YA younger or older than NA? Maybe New Adult is pre-teen (thus, new to adulthood, just starting out) and Young Adult would be teenagers. Then what? TA for Transitioning Adult, one who moved from their teens to their early 20s? Then FA, for Full Adult, which would be 30s and 40s? Then MA, for Middle Aged Adult, those in their 50s. Then SA, for Senior Adult as they move into their 60s. Then EA, for Elderly Adult, which would be 70s and beyond?
I know you're kidding a little, but New Adult is actually post-YA. It's meant to be for college-aged/20-something characters, who were a hard sell outside of certain genres before this marketing category came to be. And even now they are a hard sell outside of certain genres.
For the most part, NA is self-published, which probably leads to a lot of disdain for it because with any genre, self-publishing includes a lot of poorly written books and a few gems (because it's the slush pile). Because it's mostly SP, it's also mainly romance- and erotica-based, because those do best as SP, which means the people who look down at those genres will also look down at NA.
At least initially, NA was sort of a place where YA writers could age up their characters a little and write YA (but not YA) erotica
Now, personally, I was hoping to read books about that age group from any genre. It was an age group that many of us YA writers tried to write for, assuming that the "teens read up" meant that 18yos would want to read about MCs beyond high school, but we were told that MCs who were that age weren't marketable. So when NA started up, a lot of us were thrilled that we'd get to dust off our good old 20-something protags and try to sell those books. And unfortunately, the market just hasn't gone that way.