A couple of different reasons you might want to go with mnesia over ETS (mnesia uses ets for some of its modes).
If you don’t need any of those then of course ets will do the trick nicely
Yes, one of the downsides:)
All this said. Mnesia is a very battle-tested database, it is just that it requires a fair bit of skill to operate and that you know its limitation. There are some pretty big shops running mnesia as a major part of their infrastructure (Whatsapp, Klarna).
Why would you want to use mnesia?:
Mnesia isn’t a replacement for postgresql though, It is not an RDMS and as with any other NoSQL database you must have a use case where it fits.
Just to put my bit in:
I use PostgreSQL for anything and everything that needs to persist.
If postgresql is slow on certain queries that I cannot really make better than I either make a materialized view inside postgresql that updates on occasion, or if I do a ton of tiny little queries per transaction that all are the same (like getting permissions across connections, websockets, etc…) then I use CacheX (with a fallback in it set to querying the database) as it has auto-timeout and removal, janitor process, and far far more, including transactions and shared retrievals (it is built on ETS of course).