Are velomobiles safe? You might want to specify, safe from what? One answer might be that velomobiles are safe, but a lot of other things on the road are dangerous. Velomobiles aren't necessarily safe from trucks. On the other hand, velomobiles boast a number of safety features that have proven their value on the road, and unlike the trucks, they don't threaten other traffic.
Compared with a bicycle, a velomobile is more stable; you won't go over the handlebars and land on your head! Riding on three wheels eliminates falls on sand and ice. A velomobile can tip, but it's rare. When it does, it often slides rather than smashes on the ground. In a collision, a velomobile's body crumples and absorbs much of the impact. This has saved a number of riders from injury. Contestants at Battle Mountain have crashed their fully-faired racing bikes at speeds up to 80 miles per hour and walked away. (Links to some crash videos showing how robust these designs can be.)
The smooth body of a velomobile tends to spread out the forces in a collision with a pedestrian or other cyclist, sparing them from serious injury.
Velomobile accidents are rare, but the limited experience suggests that a velomobile provides a good combination of safety for the rider and safety for others.
A German Alleweder web page shows how a velomobile can absorb the force of a collision, with minimal harm to the rider.