There are a lot of people who play with a pure only goblin list; there are many ways to field a OnG army.
While Savage Big 'Uns are probably the most efficient unit we have, they are great at fighting other infantry. Against other unit types, I find that they struggle a bit. The other core infantry options, while not as efficient as as SOBU's are very decent, with NG's at the top of the pack. If you take Orcs of any kind, take the Big Un upgrade, its that good.
As for the Black Orc's they do well against things that SOBU's are not good against. Cavalry, Monsters, various chariot things of all types are swiftly dealt with S6(7) great weapons. The banner option is just gravy. They will lose to other elite infantry units, so some care must be taken.
Other special choices are also very good. Common trolls pack quite a punch, but must be babysat. A troll horde is a very scary prospect though.
The biggest downside of squig herds is that you need a lot of models, which is hard on your wallet. They tend to chew through anything they touch though, and are best suited for taking down enemy elite infantry.
All in all, with some exceptions (like snotling bases), there are no real bad or underperforming units in a OnG book. Pretty much anything you take will work, if you make the unit big enough.