You can make bombs out of pressure cookers. Since they are so good at holding pressure, you seal off the over-pressure release valves and then pressurize them until they burst (usually via some stupid or illegal means to begin with) then when they burst there is a ton of excess pressure. Big pressure = big boom.
They are exactly as good as any other pot made of 18/10 steel with similar thickness. Other vessels like propane tanks are much better.
Boston marathon worked because - well pressure cookers at the time didn't draw much attention to them.