The D-D reaction is less energetic and produces a neutron, and the D-He3 reaction doesn't produce a neutron. The combined reaction would release about five percent of its total energy as neutron radiation. The neutrons from D-D are about as energetic as fission neutrons, rather than the extremely high energy of D-T fusion neutrons.

They'll never run out of fuel; there's enough deuterium in your morning shower to provide all your energy needs for a year. But the need to breed He3 will put a limit on how fast they can possibly scale up. It could be that manufacturing will be slower than that anyway though.