Impact of a denied security clearance

How does getting a denial for a security clearance actually affect your career/record assuming you dont already have one? Can you still join the military? Private sector jobs? Are you banned from certain jobs for life, or at least very unlikely to get them? Why is the possibility so terrifying to some?

It will certainly make it more difficult to get a clearance in the future, since you will need to come up with time and ways to mitigate the issues that caused you to be denied a clearance.

It doesn’t have any relevance for non-cleared work and wouldn’t get brought up in an interview. It might affect joining the military since it will limit the roles you can fill.