1. As an author, responding to review feedback is a back-and-forth between my editor and the code review tool. I tend to write responses to comments while I edit code. When I'm satisfied and I've pushed my new code, I want to send all of the responses at once -- especially if two responses depend on each other. GitHub doesn't let me draft responses and then send all (unless I let the responses hang in the HTML input, which is risky).
2. On GitHub, there is no great way to update a PR with changes. Yes, I could push a new commit to the branch. But then I need to squash commits (I don't want the "bad" commits to land in main). When GitHub squashes commits, it concatenates the commit messages which is NOT what I want. (I care about my commit messages being precise and accurate.) I also can't reword my original commit message. So I guess I have to force push. But this makes it harder for collaborators to understand what has changed because GitHub garbage-collects old commits; sometimes I get the message that the old commit can't be found. On Gerrit, I (non-force) push to a "magic" ref which updates the PR for me.
3. As a code reviewer, it's really hard for me to understand what the PR author has changed since the last review. I think part of this is that by default GitHub shows the thread for a PR instead of the code. The thread view has a bunch of whitespace that makes it difficult to concisely understand what has changed. For an interface I use regularly, I really want a condensed view. Gerrit: https://chromium-review.googlesource.com/c/chromium/src/+/46...
4. Finally, Gerrit has a nice interface (like Critique) that concisely shows the PR history and what each individual is working on. (Here's a person picked randomly from Chromium code review: https://chromium-review.googlesource.com/q/owner:katzz@googl...). It's really useful for understanding what changes were made/are in progress. Again, GitHub's UI has so much whitespace that is definitely prettier but not useful for a tool that should help my productivity, not hurt it.