>>16706798 (OP)Yes. It is incredibly hard. The theory is rather simple and a competent team of physicians will have that down after finishing studies.
Putting everything in practice is hard. Starts with the simple things, like how rare 235U and plutonium are and how to seperate them to getting your subcritical mass to go critical and so on. And then you'll need to test your ideas after thinking them through really well. Every such test needs alot of work in the preparation alone, data acquisition and interpretation is hard and so on.
IMO nukes are a spook and developing nukes in current year is not worth it. There's plenty of other means to achieve your goal. The only reason organizations put their mind towards it is the nukes nimbus. They either understand NPCs still fall for the spook or they themselves overestimate the utility of nukes.
Also delivery is incredibly complicated again. Having a nuke doesn't really do much for you if you can't deliver it.