Very very few applications use mnsesia. There’s absolutely no way I would recommend it over Postgres.

I have heard the mnesia is very unreliable, which is a damn shame.

I wonder if that is fixable, or just inherent to its design.

My understanding is that mnesia is sort of a relic. Really hard to work with and lots of edge / failure cases.

I'm not sure if it should be salvaged?

I think RabbitMQ still uses by default for its metadata storage. Is it problematic?

They are in the process of migrating away from it https://www.rabbitmq.com/docs/metadata-store

can you explain why?

Mnesia along with clustering was a recipe for split brain disasters a few years ago I assume that hasn't been addressed.

I have only worked with a product that used it, so no direct experience, but one problem that was often mentioned is split-brains happening very frequently.