Most larger orgs/projects aren't going to be willing to reformat all the code in their repo in one big bang. Regardless of what promises the tool claims to make

Most larger orgs/projects aren't going to be willing to reformat all the code in their repo in one big bang. Regardless of what promises the tool claims to make, you need to be concerned about behavior changes or breakages, and most projects don't have high enough test coverage to cover everything. Depending on the size of your repo, it might not even be technically possible to make this change all at once. It's also much harder to get organizational buy-in on "let's change all existing code in one swoop" vs. "let's require all new code changes to be formatted using this tool". As I mentioned above, formatting only the diffs also removes most of the pain of any changes to the formatting tool itself that aren't diff-stable.