What fusion does is come close to ignoring the laws of engineering and economics. That's not quite the same as ignoring the laws of physics, but either of those is capable of rendering a technology a nonstarter.
Specifically, for fundamental reasons DT fusion will have much lower power density than a fission reactor, and so have to be much larger, and therefore have to be much more expensive.
Specifically, for fundamental reasons DT fusion will have much lower power density than a fission reactor, and so have to be much larger, and therefore have to be much more expensive.