Because development with Appcelerator relies on a library to access native components, you'll have to wait to access newly released native features. When beacon support was introduced on iOS, Appcelerator developers could not develop for beacons until the Appcelerator supported beacons. AirDrop, 3D Touch, Apple Pay, and Apple Watch are all native features that are only usable when Appcelerator supports them.
Any non-native framework will have the issues I've described above.
Developing in a non-native framework, Appcelerator included, inevitably leads to hard to debug issues. As you write more feature rich apps with Appcelerator, you'll consistently find yourself running into more complex problems which are increasingly harder to debug.
Appcelerator has the worst developer community I've ever seen. Threads on StackOverflow are almost non-existent. Appcelerator's hosted Q&A section is littered with poorly phrased questions with no answers. Review collected by and hosted on G2.com.