[bjackman]

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.

[oniony]

I thought Google moved off of their struggling Perforce server onto their homegrown Piper VCS.

[bjackman]

Piper is an extended reimplementation of Perforce

(Piper is piper, expanded recursively. It's so meta, even this acronym)

[steveklabnik]

And while it's not public, jj has a piper backend, so that folks at Google can use jj

(This is also why I have some confidence that jj truly is backend agnostic, it has two real backends already.)