Lots of Apocynaceae plants are dioecious. I tried to dig up info on whether Willughbeias are dioecious but failed to find the answer. I did run into a lot of cases of people growing it but never getting fruit, so that makes me suspect dioecy, or at least a need for hand pollination. Lots of reports of difficulty getting them past the seedling stage, too; be careful with your soil mix when they're seedlings. Once they get established, though, they're weeds that will smother whatever they grow on
Oh, and they have zero cold tolerance.