The vast majority of YouTubers are extremely mistaken about Fair Use. Just because you see a video that's up doesn't mean it's legitimate.
From a practical consideration, the question is whether a copyright owner is going to take action. For copyrighted works in Content ID (some music, a lot of video clips), this will happen automatically. But for works that aren't in content ID, the only way that action could be taken is if the copyright owner manually issued a strike. Not everyone is checking youtube to see if their copyright is being infringed, so infringing videos can stay up for a long time without being taken down. But there is always that risk.