Oh, no, the Witches are not even close for worst special choice. That disticntion goes to Executioners, who (despite a fantastic mini) suck hardcore. Sure, they strike with S6. They use great weapons, which always strike last. On T3, Armor save 5+ models, that means you only get one round of combat with them - the round you charge. The rest of the time the battle line will be swept by... whatever it was you needed S6 to deal with in the first place.
Sure, you can use the Banner of Hag Graef to give them
ASF, but you can't do it the easy way - by giving the banner to the unit standard bearer - because it costs too much. You have to give it to your
BSB to carry, preventing her from taking one of the other options.
Her? Well, you see, the Executioners are Khainites - devout worshipers of the Dark Elf god of murder. No Master will trust them enough to join their unit. So... you have to use a more expensive, unarmored Death Hag. By the way... she has Frenzy too, so she can be baited out of the unit.
I'll grant you that
one large unit of Witches is going to have a hard time. Several small units - especially when backed up by Hydras and characters on monsters - will draw little fire. Then they hit the battleline and go through rank & file like a chainsaw.
The problem with Hellebrone is that she is a massively expensive character on an expensive monster...
with no armor saves at all. Anything with high-strength ranged attacks (or worse, ranged poison!) will cut her down in a volley, two at most.