Nuclear fusion as an energy source has major unsolved problems. Off the top of my head:

  * The super conducting metals required for confinement randomly stop superconducting.

  * The fuels produce absurd amounts of radiation and the Helium-3 solution for that might as well be fairy dust, since even if we convert the energy global economy to helium-3 production, we will not have enough by orders of magnitude to power hypothetical fusion reactors that would handle our needs. Strip mining the moon for it is supposedly a way to get it, but defacing the surface of the moon for minuscule amounts of Helium-3 per acre is unlikely to ever be profitable.

  * The amount of radioactive materials produced from the experiments are many times those produced in fission reactors.

This is just off the top of my head. Until recently, I would have included the inability to produce more energy than we put into it on this list, but LLNL’s breakthrough a few years ago seems to have solved that. I suspect that someone with time to look into the practical issues involved in building a fusion reactor would find other issues (such as the design not being practical to use in a production power plant and thus further research being needed to make one that is).

I wonder if the only reason countries fund nuclear fusion research is to keep nuclear scientists from finding employment in the production of nuclear weapons.