I dont think there is any one type of engineer that is hard to find, rather the difficulty is finding them within your company's budget. There is a lot of talent out there, it just doesn't come cheap. The catch is that the companies that have to the budget to lock up top talent generally have large bureaucratic departments where their best engineers are really bored and/or working only 3 hours a day, and using the job to fund their lifestyle while they launch their side project.