Nuclear fission is relatively straightforward so long as you either don't know or don't care about the health risks of radiation. It's just a pile of spicy rocks at the end of the day.

Fusion, on the other hand, requires you to get center-of-sun temperatures and pressures going on to work properly. That usually requires either extremely difficult engineering processes or a fission bomb (and more precise engineering calculations but they're actually reasonably solvable).