I have an app in the App Store for more than a year now that uses LocalStorage and never had any problems with that. Data was never lost, so I wouldn’t worry about that.
Update: I now actually have experienced problems due to low memory and iOS clearing the localStorage because of that…
You have other options as well, like IndexedDB (only use that on Android), WebSQL, SQLite, or use PouchDB, which will encapsulate all of them. It so happens that I just wrote a blog post about using Ionic with PouchDB 