Even if we assume for a moment that we got it working, the factors are: scale and embrittlement. Current thermonuclear technology, theoretically, breaks even, energywise, on reactors of certain scale. There is a reason to believe that once we build a reactor of a certain size, the energy output of a reaction will be bigger than the energy spent on jumpstarting it and magnetic containtainment. embrittlement of reactor structural materials is caused by radiation. After a while, reactor has to be rebuilt from totally new materials.