What are some alternatives?
When comparing Const Apps and WPAPP.NINJA, you can also consider the following products
AppSheet - AppSheet enables users to create mobile apps instantly for both OS and Android.
AppInduce - The Next Generation DIY App Builder
Microsoft PowerApps - Microsoft PowerApps provides tools to create, customize, share and run apps.
AppPresser - Build iOS/Android apps using WordPress with AppPresser.
Jasonette - Make an app by adding JSON to this app.
Website 2 APK Builder - Convert your HTML, CSS, JS and all other web content into an Android App within seconds.