Are countertop blenders with plastic jars dangerous?
Looking at it from a materials-science standpoint rather than a physics standpoint, I agree that plastic is safer than glass.

The difference is in how they break.  Glass is stronger than plastic, for any plastic that a blender jar is likely to be made out of, and is less likely to break from, for example, trying to blend a spoon you forgot to remove.  However, glass is also brittle.

When a plastic jar breaks, the most likely break is a single crack running up one side.  The rest of the jar will flex to absorb the energy of the break and then return to its original shape.  The jar will remain in one piece, without exposing any sharp edges.

When a glass jar breaks, it can't flex the way plastic can.  Instead, the jar will shatter into two or more pieces, and unless it was made from tempered glass (unlikely) or laminated glass (even more unlikely), those pieces will be sharp-edged, making for a dangerous mess to clean up.