If you stick with the tools which actually don't abstract the native platforms (e.g. RoboVM and Xamarin), I think you will learn native development quite easily. I've taken this approach, and I've had no trouble using documentation/StackOverflow to write equivalent code in Java/C#. Since pretty much all the APIs in these tools are 1-1 with the native APIs, I've also had no trouble going in the reverse direction and writing Swift/Objective-C/Java. You will still need to learn the languages, but you won't need to re-learn the APIs (which is the time consuming part).

Personally, I see little to no disadvantage with using these particular types of tools, especially if it means you can use languages like Scala, C#, and F# instead of Objective-C and Java.