Is the National Park worth it every time? With the Globe and National Park you can pretty much grow as big as your all farm/windmill food supply allows, but is the loss of coal worth it in terms of production? Assuming access to coal (obviously the deciding factor), if you go with the suggestion of Bismarck, you loose 100% production potential from National Park. With an endgame city having at least 50 hammer output that would be a net loss in my book. Even if we're talking +50% from Ironworks (with Coal) how could you net an overall gain?
Also, on great merchants, one shouldn't forget that they effecitively count as a +1

since they cancel out one

with their +1

. Don't forget the Representation bonus either.