Bazaar-NG was never a fork of Tom Lord's TLA. That fork was Baz, which was called "Bazaar" at the time. Bazaar-NG was always a from-scratch design intended to support lossless imports from Baz.
I don't think Martin considered Bazaar-NG's storage unreliable. Bazaar-NG was self-hosting in March after 3 months of development, and Martin wouldn't have done that if he didn't trust it. His original web site warned "This is pre-release unstable code. Keep backups of any important information.", but I think this was just an overabundance of caution.
One of the reasons Bazaar(-NG) didn't switch to git's core was because git didn't provide a library. And even if it had, it would have been in C, not Python. But we also wanted something that worked with our data model. We felt we could do at least as well as git in storing data, and I've never had reason to doubt the 2a format's efficiency.