I think we have most of the actual science problems figured out. We essentially know that a sufficiently big tokamak will produce net energy. We do not yet know in detail how to build one that lasts long enough to make financial sense. ITER will probably answer the remaining engineering questions.

Of course there are more open questions around the other potential ways of achieving fusion, because we haven't thrown as much money at them as we did for tokamaks. So there is a possibility that there are much cheaper designs that also work.