I'm kind of obsessed with YouTube, so recently I did a study/in-depth analysis of the top 15 fastest growing channels for the past month. I found that every single on of those top-growing channels used no text. I think there is some kind of pattern here. For the past month, I've been adding no text to my thumbnails, and I personally think they look a lot better. However, none of those "no text" videos have done exceptionally well. All of my most-viewed videos have text thumbnails. But overall, I'd say have no text. My recommendation is to outline yourself/the subject in white, then tint the background a different color, maybe add some glow effects, and then maybe an emoji or arrow here and there.