When can you confidently rely on magic in an engineering system?

If you could have written the code that does the magic yourself.

Then the magic is just helping you have leverage.

If not, then the magic could have a very different behavior than your mental model of it.

Of course, in practice we can't peel back every layer; that's the whole point of abstraction.

But the bar is could you have understood it and written it, with sufficient time?