Regarding PR comments and line comments, file level comments have also been available from the beginning. There's a comment button above the diff view for this.
Regarding line comments being "lost", it's true that when they become outdated they can be harder to get to, but they're available. If you go to the activity tab all past comment threads are available to help make sure you've not missed anything. We're working on ways to show outdated comments in the diff view, and beyond that we're looking at adding more to our search capability for finding comments across pull requests.
For author and committer treatment, we do respect the two concepts in Git, but you're right that we don't display them clearly in the UI. We added coverage for returning both roles in the API back in 5.0, and at some point we'll probably update the UI to display it.
I'd love to hear more about what you feel is lacking in the diff view. We're doing some work there now so perhaps we can cover off your use case (or plan to already).
I have only used the installed version, not the hosted online version.
The basic PR structure is simple. Very much the same as in github.
In detail the implementation has several nasty defincies. They do not distinguish author and committer at all, which can lead to very misleading info on the screen if they are different
There are PR comments and line comments. Sometimes a file comment might be appropriate, but that does not exist.
The code viewer has limitations so I find myself using git difftool with meld or something like that in more difficult cases. Of course the difficult cases is where review and tool support would be needed most.
IMHO the biggest problem is that line comments are a one time thing lost in space afterwards. You need to know in which PR to look, actually even in which file version of the PR if the file has been updated during the review. There is no direct way to access previous review comments relating to certain code. Not sure if anybody else handles that better.