They are more easy to modify when you have custom rom and Xposed installed, but without Gapps flashed(or you just stop google services). If you have skills or you can use GG and LP, xPrivacy very well you can make the app standlone, offline if it can run online, but with the modern apps as you say it's more harder because they allways want from you to have internet connecion (even if the data can run offline) they sent logs, data usage "for better services, performance..etc." but they can spot you if you have pathed the app, they will see the log and modify it for the next update and the weakness in the code will be fixed. The hardest way is to Reverse the .apk file modify the code then recompile it and sing it. This needs some good skills.