The intent of my third point is that you should expect a decent proportion of the operations to be handled well by the ORM such that you can reap efficiency ben

The intent of my third point is that you should expect a decent proportion of the operations to be handled well by the ORM such that you can reap efficiency benefits from the abstraction. If you're spending a significant amount of time jumping out of the ORM's sandpit and into custom queries, the benefit of the ORM may be limited. And to the extent that it dictates/influences your table structures, it might be a hindrance.