I would disagree with that statement. I don't know of any instance, based on science or based on practical fact, where it hasn't worked well. What it does is it stimulates investment in the seed side from the major players growing seed as well as from some private sector folks within that country.
Canada is one of very few that has not put UPOV 91 into play. There are a couple of others that have ratified it but haven't actually made it workable. Norway is one. That being said, we're the only major grain-producing and horticulture-producing country that has not ratified UPOV 91.