MacBook v. iPad Pro Cloud Apps
The best case is when you have the same app with both OS X and iOS versions and they share state. Fantastical, a calendaring app, is a good example of this. You see a similar interface and the same calendars across devices. Spark, an email client, when it gets its OS X version, will be another. What you do on one device is reflected, almost immediately, on the other.
The key factor for both these apps is how well they interface with the underlying cloud services and merge them to present an integrated view of what you have before you. Other apps, in particular the Apple defaults, interface poorly with the underlying services, or fail to present a seamless enough integrated view.