1. It's disabled by default. You have to dig around in your phone's settings to enable APK installations, and APK installations through the specific app you prompted the installer from. And if the developer hasn't updated the app for recent versions of Android, Google will throw up a antivirus-esque "warning this app is unsafe blah blah" prompt.
2. You can't automatically update an app if you manually installed it through an APK. There are apps that can kind of do this (automatically download APK from source website on new release, notify user). But that's clunky and not suitable unless your audience is FOSS-land. Oh, and the user still has to manually click the install button for each app they update this way. No silent updates unless you're rooted.
This makes the distribution of apks through your own processes wholly unviable unless your app is mandatory for your users (I. E for work/school), or your user base is Android FOSS enthusiasts - who probably prefer that you use F-Droid (3rd party FOSS appstore) anyways