This is the same problem as to why slaves were single-threaded for so long: Statement based replication (default) is tricky to apply in parallel, since you can

This is the same problem as to why slaves were single-threaded for so long: Statement based replication (default) is tricky to apply in parallel, since you can't easily figure out ordering dependencies. In MySQL 5.6 replication is now parallel per-schema, and in MySQL 5.7 it will be parallel intra-schema.