For commits with lots of files, Gerrit's diff interface is too broken to be useful. It does not provide a compact overview of the change which is essential for effective review.
Luckily, there are alternatives, specifically local git clients and gitweb. However, these don't work when git's change model is broken by the use of git commit --amend.
For commits with a small number of files, such changes are reviewable by the use of the "patch history" table in the diff views. But when there are a large number of files, it becomes difficult to find the files which have changed, and if there are a lot of changed files, to produce a compact combined diff.
So if there are no objections, I'm going to change [[Git/Workflow]] to restrict the recommended applications of "git commit --amend", and to recommend plain "git commit" as an alternative. A plain commit seems to work just fine. It gives you a separate commit to analyse with Gerrit, gitweb and client-side tools, and it provides a link to the original change in the "dependencies" section of the change page.
-- Tim Starling