I think it depends a lot on audience. Woody Harrelson - of course his career won't be negatively affected by a Reddit AMA. But a startup whose audience is largely techies that frequent Reddit? Maybe they should be slightly careful.
But slightly careful, not terrified.
It just means not stomping in totally ignorant about Reddit culture, which demands a higher standard from submissions (including advertisers) than from regular commenters. It means avoiding sounding like a PR drone, blatant lies, and avoiding coming across too arrogant.
The Harrelson AMA seems to have blown up because a few of his responses were pretty much interpreted by Reddit users as "I am above you peons, be honoured I'm spending time on talking to you about Rampart. Did I mention Rampart? Oh, and Rampart". Which is a bit like walking up to a wild animal and kicking it in the nuts. That it seemed like a PR effort in a setting like an AMA where people expect something else but are also quite wary about the intrusion of marketeers made it doubly bad - ads are different, we know their purpose is to sell a product, and so merely trying to sell the product won't cause a stink, whereas the first time Harrelson tried to steer the conversation to Rampart, he'd already made a grave mistake.
I'd think for advertising on Reddit, a limited run asking for feedback, and being responsive and friendly in comments would be a good start.