From what I understand and can see from Google searches, de-compiling is usually the easy part, and it's not uncommon to get Java exceptions when trying to do this which I assume it related to recent changes in how the .apk files are being packaged-up and protected by the IDEs the their selected obfuscation / protection packages.
Also, unless you know the smali language these tools produce and can figure-out all of the resource dependencies, it will be pretty difficult to make much sense of the output being produced, never mind actually changing the code and recompiling to produce something useful (i.e., that runs at all).
I do know of folks that have obviously done this--usually to "theme" an app that they fancied--but doing so and sharing it with a wider audience is generally frowned-upon and is one of the main reasons why people are encouraged from only obtaining their apps from trusted and original sources like the Play Store.