[lmm]

Using a datastore for which true master-master HA is at best a bolted-on afterthought when you explicitly want a zero-downtime system is a mistake in a pretty obvious way.

Using a datastore with a black box query planner that explicitly doesn't allow you to force particular indices (using hints or similar) is a more subtle mistake but will inevitably bite you eventually. Likewise a datastore that uses black-box MVCC and doesn't let you separate e.g. writing data from updating indices.