Yeah, none of the study results I've seen have been strong. I don't trust them all that much.
I prefer snake_case and believe it probably is mildly more readable than camelCase.
The underscore comes closer to space-delimited words like we use in English, typographically speaking. It's hard for me not to think that must be easier to read that way.
Obviously I can't prove it's better, but it both makes sense to me that it would be and fits with my subjective experience.
so in this study, where the subjects were trained in snake_case, they found snake_case was better.
and in the other study, where the subjects were trained in camelCase, they found camelCase was better.
I bet if we did another study of programmers who used kebab-case they'd find that kebab-case is better.
I don't think there's any reason to believe any particular case style is better than any other. personally I like snake_case and kebab-case because I find the word boundaries easier to recognize, but that's just a preference on my part. I have no idea if it's objectively better.