I can only envision two posible risks in a GTCB investment...
1) Risk of transferring some "goat" into humans. While so far this has been a non-issue based on GTCB's studies, it nevertheless is a potential spoiler if the data ever proves differently down the line.
2) Competitive risk from Transgenic Plants. while these are still far behind GTCB, the concept is a lot easier to digest (no pun intended) and could result in much better controls and social acceptance.
Otherwise, I consider GTCB to be a very low risk investment...