I ought to try syncrepl before talking too much, but anyway:
Quanah Gibson-Mount writes:
It sounds like it gracefully solves the ability of keeping both master and replica configurations around for the most part. What still remains sticky is ACLs. There are plenty of valid reasons for the master to have very different ACLs than the replicas do.
And things like <authz-policy> and <allow>. Security settings, if you run a master inside a well protected subnet and partial slaves on more open ones.
Master and slave can share some but not all databases, and you might want to replicate config of those they share - but this way the database numbers will differ. Might not share all related schema either.
<threads>, cache settings, <argsfile>, etc. if your servers run on different OSes. Which can be useful so that if an OS-specific problem hits one server, others are in no danger.
I'm sure there are good reasons for plenty of other things to differ, while config-replication could still be useful. Depends on how flexible partial config-replication is intended to be.