First, this might be off topic, I'm not clear on how far this site goes into marketing concepts. If its off topic just leave a comment and I'll delete.
I'm preparing to release an app centered around video streaming which allows users to communicate (audio) and stream video between their devices (for example, videos stored on their device). I've been contacting video advertising providers with interests in publishing pre-roll video ads before videos streamed between users on this free app.
The first response I received explained that for reasons including "brand safety" they weren't interested in supplying ads to applications involving user generated content or social contact/messaging:
I don’t think this will work for us right now because there is no control over the video that’s being streamed. It opens us up to concerns about brand safety and running on user generated content. We also do not work with social or messaging apps.
In order to understand this concept and what it means for advertising opportunities with my app, I'm curious:
Why might an advertisement distributor not want to work with user generated content or social communications? What implications would it introduce for the advertiser in terms of brand safety or advertising effectiveness / returns.