The Waterford bit can be controversial, but what can't be argued is that it is a fairly popular bit in the hunter ring. To the outside observer it might appear to be an uncommon equipment choice, but D-ring cheeks can hide many a different mouthpiece!
The Waterford is a four-jointed bit, unlike the two joints of the french link or the one joint of a regular single-jointed snaffle. Because there are four joints, the bit is very flexible. This can make it a good choice for horses who tend to lean down onto the bit because there is no solid piece for the horse to hang onto.
The allegations of harshness likely have to do with the possibility for the mouthpiece to wrap around the shape of the lower jaw if too much pressure is applied without release. Any bit will become harsh if used by harsh hands, but special care should be taken with the Waterford to avoid its use by uneducated hands not because it is an inherently harsh bit, but because its construction has the potential to offer the horse no relief from pressure in the case of heavy hands. Because many horses will learn to lean on a rider's heavy hands, rider education can sometimes be a better solution than a switch to the Waterford bit.
Its popularity in the hunter ring likely lies in the typically low way of going favoured for hunters, which can occasionally transition from long and low to downright heaviness. A Waterford can provide a gentle reminder to a heavier-travelling horse to lighten its forehand when used by a considerate rider. Combined with the light contact favoured in the hunter ring, the Waterford in theory can be a gentler choice than a bit that would require more pressure, constant reminders or increased leverage.