It’s not risk but cost and also difficulty in ground testing safely. What changed is they may not test them on the ground but in orbit. Just design it very conservatively and launch to a safe orbit and test there.
Technology can just progress, nothing massive needs to change. DARPA sees that the time is ready to advance this technology once again. They will test it first at very small scale. The purpose is deep space space force robotic vehicles being able to make lots of maneuvers (to avoid ASAT? To do multiple missions? Changing orbit to avoid detection?) with high thrust, ie quickly.
Technology can just progress, nothing massive needs to change. DARPA sees that the time is ready to advance this technology once again. They will test it first at very small scale. The purpose is deep space space force robotic vehicles being able to make lots of maneuvers (to avoid ASAT? To do multiple missions? Changing orbit to avoid detection?) with high thrust, ie quickly.