There's also no control over the content. As an advertiser, I would be worried with where my ads may end up. With a traditional publication, I generally know the type of content that will come out and their policies.
For your publication, the writers will keep writing as their job, but if you upset the content generators at reddit, you'll lose your content source.
In general, monetization of a publication is a more proven model than monetization of user generated content.