Is it important to allow comments on Reviews? I can imagine people commenting on Reviews to complain about it just because they have a different opinion and want to argue.
Also, I have seen several Forums with a comment function and all of those ended up being a huge mess.
Threads are pretty straightforward, everything new will show up at the end, but comments will bring chaos into it. Suddenly any thread can have new content right in the middle of it and users would need to check the entire thread over and over again to not miss stuff.
Imagine someone talking about an issue and the solution is in a comment to that specific post, nobody will see that comment, because they will only see the latest post.
Besides that, notifications and alerts are always bugged with comments.