I have no opinion on whether JJ is good but I am pretty confident your impression about the reason for its shortcomings is wrong.
Google has a basically tolerable and pretty easy-to-learn Mercurial-based frontend for its bizarre legacy Perforce system.
Everything about JJ screams to me that it's been created as the passion project of someone who really wants to build a better VCS, making it compatible with Git was necessary to give it a chance of adoption, and making it compatible with Piper (Google's Perforce thing) was a way to get it funded as a potential benefit to Google.
Top-down Google engineering would never produce a project like this IMO.
Google has a basically tolerable and pretty easy-to-learn Mercurial-based frontend for its bizarre legacy Perforce system.
Everything about JJ screams to me that it's been created as the passion project of someone who really wants to build a better VCS, making it compatible with Git was necessary to give it a chance of adoption, and making it compatible with Piper (Google's Perforce thing) was a way to get it funded as a potential benefit to Google.
Top-down Google engineering would never produce a project like this IMO.