Different devices require different acquisition techniques. The different versions of iOS require different acquisition paths, each having its own vector of attack. While in Apple's sector, we don't nearly see such a wide diversity of devices and heavily-customized versions of the operating system (each with its own vulnerabilities), there are still a lot of devices available.
iOS is a closed operating system that's only supported on Apple-manufactured hardware. As a result, we don't have to deal with a dozen different manufacturers and thousands of models. By Q2 2015, Apple had released ten iPhone models, five models of iPod Touch, six full-size iPads, and three models of iPad mini.
While general information and up-to-date specifications of Apple hardware can be viewed at https://en.wikipedia.org/wiki/List_of_iOS_devices , there are several things not mentioned in the official specs that can make the device more or less susceptible...