iPad is 'that' hard because it is non-trivial to take advantage of the new screen real estate. Often-times you must implement a completely new navigation model because the stock split view controller is rarely the navigation model you want to use.

iPod touch is generally easier. It usually comes down to gracefully degrading features. Much care must also be taken to ensure that the app performs well on the iPod touch models which are often running much weaker hardware specs. The reality is that sometimes devs will only test on their personal iPhones and forget to test on older-gen iPod touches, so the iPod touch experience can sometimes be less than ideal.