But Scylla doesn't offer the same consistency guarantees as Mongo, right? You can shard pretty wide with Mongo and still read your own writes, while my understanding is Scylla is eventually consistent?
Scylla has "tunable" consistency[1][2][3], so it's possible to get consistency without too much of a performance hit, and still be resilient to some failures.
On top of this, ScyllaDB also has "Lightweight Transactions" (LWT) that use Paxos to do a compare-and-set (CAS) read-before-write. This is important for strict linearizability. You can think of it really roughly as "single partition ACID."
They are also implementing a Raft infrastructure upon which they will be able to do strong consistency transactions in the future.