This is the main issue and I don't want to make it any more complicated. But I do want to explain something because people hear arguments between fusion scientists. Within the field no one really doubts that ITER is going to work (there's a few, of course, but the majority don't doubt). The thing they are arguing over is if it is needed. The big reason for ITER is not to generate power but to demonstrate that sustainable fusion is achievable. The idea is that if there is a demonstration people will stop shying away from it and investing would skyrocket (a government would have to be dumb not to substantially invest in PROVEN fusion devices). The counter argument is that there is enough funding and that they will beat ITER to the punch with small scale reactors (much closer to the path for devices that would provide actual energy to our grid). My take is "don't put all your eggs in one basket."