They can be decent against single target, but it's not immediately evident how. That's probably where the notion comes from. But that's only the case because it is a natural weak point of the spec. I mean it wouldn't be terribly balanced if they just facerolled everything right out of the gate, would it? Every spec has (and should have) a downside, and I don't think it's terribly controversial to admit that single target is that of melee necros.
Before I lost my old saves, I had a setup with a dagger/shield that managed to be pretty decent against single target with just parasite. Melee necros with dagger/shield are underrated, imo.
That said, my old unholy Paladin with Malthael's set outclassed my melee necro in just about every way, so I stopped looking into min/maxing this particular build. So... yeah, there very well could be ways to make them top tier in terms of single target DPS. If you know how, feel free to share your approach (and/or build) and put this misconception to bed once and for all.