I have never turned on non skippable adverts so I cannot speak from proper experience but you need to weigh up the pros and cons of having them enabled, you could lose views on the video from people clicking away when they need to wait 30 seconds and your audience might not like it to where they leave but on the other hand some of them will stay expecting a really good video, I know personally when I see a video with a 30 second advert I have high expectations of the video but if it doesnt deliver after that I am out right away. For me I wouldnt bother unless I had a massive audience and I am talking a good couple of million and even then I would likely only do it on some exceptional videos.