They kind of are if you understand what role these ingredients play and why. I was able to make an easy substitution in an oatmeal cookie recipe because the nuts I was adding were primarily bulk as was the oats so the substitution worked out fine. If, on the other hand, I replaced oats with more raisins that might not have worked because the raisins would add more moisture.

You can replace like for like provided they are doing the same job