The standard way of frustrating decompilers for Android applications is with heavy obfuscation using ProGuard [1] or DexGuard. [2] IMO, DexGuard is a real pain in the ass to reverse around. If you aren't dealing with heavy obfuscation, decompiling APKs is trivial using jadx. [3]

In general I have found that most smaller app developers don't obfuscate at all, and often you can find hardcoded keys/secrets in these smaller applications.

[1] https://developer.android.com/studio/build/shrink-code.html

[2] https://www.guardsquare.com/en/dexguard

[3] https://github.com/skylot/jadx