The major problem is that learning to control even a relatively small nuclear fusion explosion and properly channel it into thrust has never been done. Nuclear fusion reactors being tested for power plant usage have only begun to reach break even energies. This means that the amount of power produced is barely equal to the amount of power required to contain the explosion. Did I mention that these reactors are really big? The Charger-1 weighs in at about 50 tons and that’s just the engine. The researchers are proposing that any spacecraft using these engines would have to be built in orbit because total spacecraft size could exceed 500 tons. That’s about the same mass at the International Space Station in orbit right now. Even using 3D Printing techniques to manufacture most of the spacecraft, that’s a very large amount of 3D Printing polymer to transport into space by conventional means.