Hydrogen bombs may not be harder to design because the fusion step has so much room for error; IIRC, André Gsponer and Jean-Pierre Hurni's technical report _Fourth Generation Nuclear Weapons_ http://web.elastic.org/~fche/mirrors/www.cryptome.org/2014/0... says that hydrogen bombs are actually much more reliable and hard to get wrong to the point where it's surprising that recent nuclear states like India or Pakistan don't start with h-bombs and work their way down to the trickier a-bombs.