Proliferation and Terrorism
With regard to the risk of proliferation and use by terrorists, the relatively simple design is a concern, as it does not require as much fine engineering or manufacturing as other methods. With enough highly-enriched uranium (not itself an easy thing to acquire), nations or groups with relatively low levels of technological sophistication could create an inefficient—though still quite powerful—gun-type nuclear weapon.
Read more about this topic: Gun-type Fission Weapon