I agree with this! That's why I opt to use a frame from inside the video usually.Anything which makes the viewer click and than keeps him/her watching.
The worst thing which may happen is viewer clicking on your ultra- fancy thumbnail and after that almost immediately clicking away when he/she see that there is no any good relation between thumbnail and video content.
100% a custom thumbnail is better. Unless the person is a subscriber the 2 things the viewer will see first is the thumbnail and the title making both very importantWhats better, making a custom thumbnail or choosing one from the video?