- Will not rewrite an immutable commit, of course. (Though you can tell it to, and for my personal repositories I sometimes actually set immutable() = root().)
- Will not do anything to diffs if it's uncertain where they should go, either because that file wasn't edited in a previous mutable commit, or because it was edited in multiple sibling/cousin commits. The latter is likely to happen with mega-merges.
You can use --into to restrict the set of target commits, if you already know what feature branch you're updating.