I have a vague feeling that those who use Oracle are using it due to circumstance or corporate necessity.
Oracle had its tentacles everywhere in that company. I think even they wanted to stop using it but it was just integrated to too much.
Including horizontal sharding and vertical replication out of the box?
There are absolutely mongodb deployments out there that are at sufficient scale that they genuinely need those features in any storage backend, but I suspect the vast majority of them only need those features to work around mongo's mediocre straight line performance.
How close this comes to counting as "almost all" is of course highly arguable.
And lose all the data in a single failure?
I'm very confused why you might think that could "lose all the data" from a single one of those nodes failing.
Getting operations right is tricky no matter what you're doing/using, and you always have to learn your backend's idiosyncracies no matter which one you choose.
A simple wrapper in a language like Go or Rust is sufficient to surpass Mongo performance.
Personally, I have shifted database operations behind a GRPC service that uses Go language and PostgreSQL back-end. Allows me to customize the data store to suit the requirement.
PostgreSQL does not get enough love in this world.