I don't think the skill has to be inherently difficult (although difficulty might have a direct correlation with ease of access to documentation, which means rare skills will always be difficult because no one has an incentive to maintain public/low bar access to documentation). The difficulty might lie in understanding or predicting that a particular skill will be rare and valuable in the short/medium term. Being an early adopter is risky, but if you make a winning bet the world will be your oyster.