Some abstractions are basically perfect and don't require you to understand the layer beneath them at all. Some have that as their ideal, but are imperfect, and sometimes require you to understand a few details of the layer below. But others - the best example that I know of is SCSS - aren't even trying to eliminate the need to understand the layer below. Everything you need to know to write CSS, you still need to know to write SCSS, and then SCSS adds more on top. It's a convenient tool for power users and I wouldn't want to be without it, but for a beginner, using SCSS instead of CSS means you have strictly more to learn; I'd suggest that a newbie on my team not touch SCSS until they've done a few days' work with raw CSS.

While less clear-cut than SCSS/CSS, it seems to me that ORMs/SQL have a similar relationship. Frankly, without knowing SQL, you can't hope to competently use an ORM; you won't know what sort of queries and updates it's capable of or what its performance characteristics will be, let alone more subtle stuff like what indexes to create or how the database's transaction model works. But even once you DO understand the SQL layer, getting to grips with how ORMs work is a major additional hurdle.

For that reason, while I am comfortable using an ORM for projects I work on, I wouldn't recommend that a beginner do the same. If I did, I'd be doubling the amount they have to learn, and creating a risk that they'll give up in despair trying to figure out how something poorly-documented works at the ORM layer because they don't have the knowledge of the underlying SQL layer to intuitively guess what their ORM code must be doing under the hood.

It's perhaps more radical than anything I believe, but I think you can reasonably go further and make the case that the extra learning curve imposed by ORMs is not worth the minor convenience benefit they offer to the proficient, and that for that reason, it's generally better not to use them. The fact that they leak the entire layer underneath them is part of that argument, but not all of it; the other part is that the layer they build on top of that is difficult-to-learn and only adds a small bit of convenience in exchange.