It's hard to show with any accuracy how likely a filesystem is to not break when the SHTF or something weird happens, or if they've handled all the weird corner

It's hard to show with any accuracy how likely a filesystem is to not break when the SHTF or something weird happens, or if they've handled all the weird corner cases, with any kind of automated test. For that you have to dig into the methodology, look at the code, look at user reports, etc. But you can get a pretty good approximation just from the philosophies and attitudes of the engineers and what they're talking about. The talk I just gave at the Rust for Linux conference was all about that - how do we make the system debugable, the community aspect of how we respond to bug reports and talk to users, the prep work for the Rust conversion and formal verification and how we're approaching all that. Reliability doesn't come out of nowhere, "all bugs are shallow with enough eyeballs" really doesn't apply to filesystems. You just have to plan for it, come up with a methodology, and do the work.