As someone who's completely new to the website, I mainly registered to leave a review on a game I really enjoyed. Did that on the comment section and then wanted to check out, if there's anything more I could do for the game - like rating it, etc. And then I had to google a bit to figure out where the difference between commenting and reviewing is ... I'd really like to rate the game, but then I'd have to send my review twice to the developer and also the rated review wouldn't be public? That's just so confusing, honestly.
I think that both leaving a public comment/review and sending a private (for example for detailed bug reports) message to the developer would be viable.
However, I'd find a merged system nice, where when I want to comment on a game, I get asked if I want to just comment (maybe for a quick question n stuff) or leave a rated review (Plus maybe the option if I want this to be public or private?). Both ideally with the possibility for the dev to reply properly.