The technology to make small, compact, high output thermonuclear weapons isn't out in the open nor is a lot of the technology used to reduce the CEP of ICBM warheads.
The big thing we're missing for modern weapons is the nature of aerogel between the primary and secondary stages. You'll still get a nice boom if you screw that up.
Basically we know why we want it (optimally igniting the second stage using the energy of the first stage), but we don't know exactly what it's made out of or the process for making it. We know it's gnarly, toxic stuff though.
