John:
I have a hunting buddy who is about 7 years into his second WHPG after 12 good years with the first. Both have been great dogs in the field for partridge and woodcock and very game on ducks in warm or cool water. (And that's coming from a guy who generally prefers flushing dogs for upland work.) Cold tolerance was an issue in the late season here where we often hunt ducks on the ocean with snow on the ground and ice in the water. They were game, but shivered even before they got wet and it's clearly not the dog for those conditions. Both were fine on the salt in the early and mid season, and good hunting late season partridge in the snow and cold so long as you kept them moving.
Both had very sweet dispositions, too. The dogs my friend had were only available to owners who agreed to at least some level of training as versatile dogs, including upland, water work, and even tracking large game. That may not be true for all breeders, but it seems common for Griffons I've seen in Maine.
Another friend had a rescue Griffon from the pound they got about age 2. It was a sweetheart, but compared to a Springer or Lab with similar background didn't seem to have a lot of interest in birds. They didn't hunt so this was no issue. That may mean early exposure to birds and training is important for the breed, but I'm judging based on a sample size of one untrained dog from uncertain origins. They had no problems teaching it basic obedience despite a clear lack of that background when they got it, and as I said, she was a sweetheart once she had a little training.