What happens if this security update installation fails?

The app store installer got stuck while installing this. This creates all sort of uncertainties:

1. If I quit the installer, will it properly install the security update on a future launch?

2. If the update is not properly installed, will future updates fail?

3. Do I have to track down every single issue fix manually to tell if and when the update is applied?

4. Why is the deterioration of Apple software quality and basic quality assurance accelerating?

That progress bar has been in that identical state for hours.