Different downstream effects man. Fina does have the potential to cause ED though, and even potentially destroy someone's ability to produce DHT at all, to the point that even at greater than 1000 ng/dl of test they can't get hard, and experience depression.
But DHT performs mostly virilizing effects, while (of course) contributing to other functions in the body, test can actually fulfill those roles if the dosage is cranked high enough. The guys who no are no longer capable of producing DHT have seen positive results from supraphysiological dosages of test, since any androgen will bind to androgen receptors, but DHT has an insane binding affinity, while test's binding affinity is pretty weak.
So, DHT is thought to be mostly utilized during puberty, while a tiny portion needs to remain in old age, and it does actually decline as we age along with test. Basically, Finasteride is reducing an androgen we have in overabundance that we don't need nearly as much of, while the anti androgen effects of nandrolone directly reduce hormones we actually need for a healthy libido.
