the JSWT was a engineering problem. the physics behind it are very sound, it was just complicated.
The physics behind the ITER are not that sound. IMO the one AFTER the ITER can be treated like a james webb! If ITER proves the physics, then you just need a deliciously complicated engineered system to make it high Q.
The JWST has questions like, will this linkage survive the vibration. is the material that the mirrors or whatever are made from not going to crack.will something not get cold welded in space and jam the mechanism or will some guy wire tear up.
The ITER has questions like "will this thing even possibly hold plasma". I don't think with the amount of knowledge we have on telescopes that you could call the JWST a physics experiment, just a engineering problem. The ITER is a physics experiment. I don't think they can even say that it SHOULD work conceptually. More like might.
Now I do agree that there are sub components of the ITER that are similar to the james webb space telescopes. The vacuum system, the heat load, the power supplies, the RF heaters, the piping, magnets and waveguide. Its very complicated and tricky but conceptually those subsystems work to bring the correct 'bias' to the reactor if you give the engineers enough time, its guaranteed to be in spec. But even if you build all that complicated stuff to spec, it might not work because there is basically a dynamic control problem with a unknown specification at the core. There is no guarantee that those sub systems will be the correct control parameters for a fusion reaction.
I would love to know what the anticipated possibly failure points are for it not working if you build everything right.
At this point, it looks like if you got a blue print of that design, it would have little comic book diagrams that say "maybe one of these will work here".
From reading the wikipeida page, with the section on additional heating, with how many... possibilities there are, I half imagined a crew of like 10 guys with acetylene rose bud torches heating up some pipe going into the reactor to try to get it to start lol