Some apps might be more difficult to hack than others. They might have implemented cheating protection systems to check for uncommon changes or hacking attempts.
One protection mechanism would be for example encrypting Shared Preferences. Or the app might also use checksums in order to verify the integrity of the file.
Hacking Android Online Games?
While this might already be tricky (or for casual users impossible) to bypass, it becomes really complicated for online games. Apps that require you to have an online account will most definitely only store the login credentials and some cache data on your device. Everything else is going to be stored online. Cheat Droid only allows you to modify local data. You will not be able to access online servers. Cheat Droid most probably will not work here.