When we started working on Breezy, it was mostly as a way to keep Bazaar working going forward - in a world where Python 2 has mostly disappeared in favour of Python 3).
Since then, we have also made other improvements. In addition to Python 3 support, Breezy comes with the following other bigger changes:
Breezy bundles most of the common plugins. This makes the installation of Breezy much simpler (pip install brz), and prevents possible issues with API incompatibility that plagued Bazaar.
Bundled plugins include: grep, git, fastimport, propose, upload, stats and parts of bzrtools.
>120 fixed bugs
Since Bazaar 2.7, lots of bugs in the Bazaar code base have been fixed (over 120 as of March 2019). We've also started an effort to go through all bugs in the Bazaar bug tracker to see whether they also apply to Breezy.
Native Git Support
Breezy now supports the Git file formats as a first class citizen; Git support is included in Breezy itself, and should work just as well as regular Bazaar format repositories.
Bazaar has always had a higher level API that could be used for version control operations, and which was implemented for both Bazaar, Git and Subversion formats.
As part of the work to support the Git format natively, we have changed the API to remove Bazaar-specific artefacts, like the use of file ids. Inventories (a Bazaar concept) are now also an implementation detail of the bzr formats, and not a concept that is visible in the API or UI.
In the future, I hope the API will be useful for tools that want to make automated changes to any version controlled resource, whether that be Git, Bazaar, Subversion or Mercurial repositories.