From my personal experience (which probably has to do with working in industries where incoming data has a wide array of clients who need it ASAP) - the biggest challenge with any distributed DB system is the problem of "reading your own writes" and how the system approaches it. Not to be confused with tx isolation levels.
It's a balancing act between two extremes - locking everything down and ensuring the tx has been committed on all nodes/propagated to all consumers on one hand and sending an "ack" back to the client with a loose promise of eventual consistency on the other.
It's a balancing act between two extremes - locking everything down and ensuring the tx has been committed on all nodes/propagated to all consumers on one hand and sending an "ack" back to the client with a loose promise of eventual consistency on the other.