I'm not a cover channel, so I've never looked into these things for myself. However, I've seen many posts on YouTube's official forum about the subject. There are thousands of complaints and unanswered questions on the matter. You upload a cover without any type of license and nothing happens. Someone else does a similar cover and gets blocked or even receives a strike. Sometimes the whole year goes by without a problem and then all of a sudden people wake up to a terminated channel without warning, because multiple strikes were issued overnight. Apparently YouTube's content ID is not exactly a consistent automated solution to clear the confusion.
I only upload original compositions or pieces that are public domain and still receive copyright claims from time to time. Last time, I received multiple copyright claims for Brahms Rhapsody within minutes. I file the dispute of course, and eventually they get removed, but still I don't understand what is it they're claiming exactly?