Google explain that their tests indicate similar performance results for text ads on both Google Search and YouTube.
From these tests, Google have made the decision to incorporate YouTube into the Search Partner network.
For advertisers, this means when you opt-in your text ad campaigns to search partners, your text ads will be able to appear on YouTube search results pages when users enter keywords that are relevant to that campaign. It’s worth noting that this feature is only available on mobile. At this point, your text ads only appear in the mobile search feed on YouTube, and it is worth noting that ad extensions are not yet supported to appear on YouTube mobile search results.
Google explain that “This tells us that, even in a video context, text ads shown to the right person at the right time, matching the right intent, can be really effective at driving conversions.”