First, the 50K problem-- that's about two-thirds of a short novel in most genres. If you write a complete story at 50K, and it's a mainstream adult genre novel, something will be missing. There are lots of folks who talk about adding material to a short novel, but unless you're completely re-conceiving and rewriting, how the heck do you do that? Very hard to do without tossing out your 50K, or at least treating it as some kind of über-outline for your story.
So Nanowrimo leaves you with a bit of a pig in a poke. It's not a novel, and it's not something that can be easily turned into a novel.
Second, the bad habits. Like thinking you can write a good novel quickly. Heck, any novel quickly. Yes, there are folks who can do it. I don't know of any, but I'm sure they are out there. I'm sure that somewhere there is a writer who has successfully written and edited a novel in one month that went on to critical success. But I don't know of that person. I've never heard that story.
My worse concern here is that writing a 50K story gives a new writer a false sense of the structure, pacing, and complexity needed in a novel.
Maybe what the Nanowrimo needs is another month. Take October. Two months should give a writer enough time to produce a rough draft of the proper length, if they hustle. And then they need another six month rewriting festival. But that shouldn't start right after the completion. Put the novel away for a couple of months. Then start on the rewriting. Then you'll need time to get the novel to your trusted readers, get some feedback, do some more rewriting...
Maybe we need a Nanowriyear.