[subject] + 不外乎 + [noun] / [noun]、[noun]
[subject] + 不外乎 + 以下几种 / 两个 / 这几点
不外乎 says that something falls entirely within what follows and nothing else: "comes down to", "is no more than". What follows is a short list, or a number naming how many items there are. It is written Chinese.
The subject is usually a category – reasons, methods, topics, activities – and 不外乎 sorts it into its few familiar members.
无非 is common in speech, takes 是 easily, and is the word for someone's plain motive: 无非是想… "all he wants is to…". 不外乎 is more written and lists the contents of a category; it doesn't explain what a person is after.