Can Appium be used for all my tests?

This is an implied question in this question. The answer is No (in general). As I said before Appium is not suitable for all types of tests you might want to write (this depends on the functionalities you need to cover). There are some scenarios that can be difficult to test and some of them are so platformed specific that you will need to write some suites just for Android or iOS for example.

Remember that you can always get to do something no matter how hard it is, so you can test all your difficult scenarios using Appium, but always keep in mind one question: is it worth the time and the pain? Having Appium testing some scenarios leaving a few tests to other approaches is fine too! The world is not black and white!