As others here have hinted at, while NNT and NNH are both extremely useful concepts, they (deliberately) leave the consideration of whether a treatment's level of effectiveness, or the magnitude of a harmful side effect, subjective. When, after all, should a negative side effect be counted as "harm"? This is especially true for NNH; there are usually far more possible (helpful or harmful) side effects than the one intended remedy for the ailment, for a given drug.
As an example, in one study, for elderly patients the NNT of a number of (admittedly different) sedative hypnotics was 13, while the NNH was 6 [1]. From that alone, you'd probably rightfully avoid using them! But for certain populations who have chronic insomnia, the benefits may still outweigh the risks.
Of course, there are ways to (less subjectively albeit still subjectively) quantify harm, and these get used to keep "harm" comparable to "treatment". But it can definitely be more of an art than an exact science.