The ideal working terrier, if there is such a thing, represents a set of balancing points. Above all it must be small enough to easily negotiate an earth and follow the quarry wherever it goes. At the same time it has to be large enough and strong enough to spend an entire day in the field, often in miserable weather.
Another factor with very hard dogs is that many of them are mute or nearly-mute. A dog without voice is a serious liability because you never know if it has found its quarry or is merely stuck in the pipe.
It is worth remembering that a fox, groundhog or raccoon can see nothing underground. Nada, zip, zilch. The darkness is complete and the picture for both dog and quarry is pure blackness. For an experienced dog, this is less of a liability than for the quarry. The dog, after all, knows what a groundhog is, what a fox is, and what a raccoon is. This is not the dog's first rodeo.
For the quarry, however, this is probably the first time it has encountered a dog in its den. It has no idea what to expect, and its first inclination is to flee -- a response that rises rapidly if the dog is barking and growling just a foot up the pipe.
It is very rare for a fox, groundhog, possum or raccoon not to flee from a baying dog, as standing to fight is a very maladaptive strategy for a small animal. Unless there are young in the den, there is nothing in the pipe to defend, and in most cases a fox, raccoon or groundhog will simply abandon their young to the dog since self-preservation is a genetically encoded response.
A dog that goes in silently and grips the quarry is not allowing the animal to flee, but forcing it to stand and fight. While some terriers do learn to grip a fox by the throat and push it to asphyxiation, most do not, and most dogs take a pounding if they try to grip in every situation -- a bit like a boxer who knows only how to slug. Such fighters do indeed have wins, but they do not have great careers.
A dog that approaches all quarry in every pipe as if it can muscle its way to success is a dog that is going to take a beating over time. Under most circumstances, a groundhog cannot be killed underground -- they have no necks and skulls as thick as a breakfast skillet.
A raccoon is another serious animal with very good canines and a crushing bite. A fox has a very light build, but sharp canines which can leave deep muzzle punctures and take out an eye. If the rip is particularly serious, it may be two or three weeks before a dog can see action in the field -- a lot of time away from work, especially in a foxing season that may last no more than eight weeks in its prime, and offer perfect weather conditions for just a fraction of that.
The baying dog, on the other hand, is like a boxer with a full array of skills. If the dog understands its job -- and the digger understands his -- it will use voice and grit (and yes, this means the occassional use of tooth) to move the quarry to a bolt or a stop end, at which point the owner will dig down and either release or dispatch the quarry as required.