I have not used foreverboard . In fact, before your post I had not even heard of it (new products come to Montana very slowly)
After reading about it , I would buy this for any new construction.
They don't seem to want to give out prices... I'm sure its not cheap, but if it is impervious to water damage or mold, it would pay for itself in the event of flooding.
Well, after a bit more digging, it seems they have some drawbacks in high humidity/ moisture exposed usages, that they are hygroscopic and, although they cannot mold/deteriorate due to water themselves, any iron or wood in contact/proximity will eventually leach moisture off the boards and corrode/mold.