Is MySQL Cluster just for in-memory databases?
In NDB Cluster, data does not have be memory-resident, but I think the indexes do.

This technology is good for high availability and high activity.  It is not for the typical RDBMS application.

NDB's Replication is based on "eventual consistency", where in you can do conflicting inserts/updates, but define the rules of which one wins.

While it is closely related to the rest of MySQL, NDB Cluster is not identical; there will be some coding changes if you migrate.