Construction on ITER has not yet begun. It's not so much that there are many 'we don't know how to do that' problems, but rather problems of the 'we can't decide which solution would work
best' sort. The engineering design is expected to be frozen next year. This is NOT vaporware.
When I was in grad school
, I took a plasma sequence taught by one of the ITER stability physicists. I was pretty surprised to learn that the expectations that were sold
to the politicians are very very conservative compared to what is expected. I thought this pretty foolish, given what I know about public cynicism towards science, and also since I'm pretty sure the (cynical) politicians themselves are expecting that the physicists are overselling
their proposed device rather than underselling. That is after all what politicians
do. Scientists are probably among the most cautious and conservative people in the world. It's no accident
that accelerators and space probes last far longer than their 'expected' lifetimes.
ITER is expected to operate at the lower edge of what a viable commercial plant would need to produce, right out of the gate. The principal problem is that the physics is too hard to do theoretically (in physics speak, this means 'mathematically'). We have empirical operational data points for smaller D-T fusion reactors, we can fit them to a curve, and we can project
where we expect ITER to operate. This is why 'knob twiddling' is so important. As soon as the thing is turned on, they'll start collecting data points, and the physicists will likely know very soon exactly how to make a viable power plant. After that, it's all about 'how much is good enough?' and 'should we start building commercial plants now, or wait and try XX at ITER'?. That phase is expected to last ~20 years, but If I had to guess, I'd guess that the first commercial plants will open several years before ITER the experiment
In the meantime, fission works just fine. In fact, more than just fine. Honestly, we could fiddle with ITER for thousands of years, and fission would still be just fine.
Fun side-note: ITER is designed to be a 'burning' plasma fusion reactor using deuterium and tritium. The D can be scooped out of the ocean, but the T needs to be made, so it's not completely trivial to get. D-T reactions have the largest 'cross-section' which is a fancy way to say 'they're easiest to do'. However, D-3He has a cross section one order of magnitude (10X) smaller, but with an output energy at the peak almost a whole order (60keV for D-T, 450keV for D-3He) higher. This is a pretty big deal. Where do you find 3He? The moon.