Unfortunately when I look through my old Favourites playlist, which at some point reached the limit of 5000 videos, I can see how many of those videos are now private, deleted, or blocked in my country. The worst part is that in many cases I can't even recover the video title, so I have no idea what has been lost. A possible solution would be to store the titles separately, but I didn't think about this while I added stuff to my collection. I do agree though that the unique, unchanging URL is a huge boon, when I look at the situation in my browser's bookmarks for comparison.