DU is not entirely benign. Relatively is another issue entirely. An imbeded DU fragment is more dangerous to long-term health than a more inert (eg, steel) shrapnel fragment. On the other hand, it is not inherently dangerous to handle or be around.

On the topic of "mini-nukes", one should not confuse nuclear artillery munitions, which have been around since well before 1968 (when I first trained with them) with penetrators ("bunker busters"). The 155mm artillery round is designed for area-denial and large targets, like the Soviet attack through the Fulda Gap that we were all anticipating (and training for) throughout most of the second half of the twentieth century. A penetrator round is entirely different, and is designed to reach an underground, and hardened (as opposed to just buried) bunker and destroy it with a minimal amount (which is different from none) of released radioactivity. IMHO, it is unnecessary and worthless, since it depends upon completely accurate targeting information, which events in Iraq and Afganistan demonstrate that we rarely have.