> to know how a specific ORM composes queries targetting a specific database backend, however, is where the magic falls apart

I've never found this to be a particular problem. Most ORMs are actually quite predictable. I've seen how my ORM constructs constructs queries for my database and it's pretty ugly but also it's actually also totally good. I've never really gained any insight that way.

But the sheer amount of time effort I've saved by using an ORM to basically do the same boring load/save pattern over and over is immeasurable. I can even imagine going back and doing that manually -- what a waste of time, effort, and experience that would be.