1. There are large differences in screen size between the various devices. You need to take that into consideration when designing your app. This is why there are dozens of simulators - you need to test your app on all of the various screens.
2. You say "seamlessly across all devices" - the answer is no. All devices do not have all of the features Some have BT, some don't. Some have WiFi, some don't.
Now, you also say "etc" - what other features do you need? Ask specific questions, because there is no "etc" when dealing with BB API's.
One other comment - you also need to be very cognizant of the network issues from carrier to carrier. If you need HTTP or sockets, this will be your biggest pain.