Basically, when penal substitutionary atonement becomes your sole window on salvation it reduces to "fire insurance," escaping from the judgment of hell. Relatedly, Dallas Willard described penal substitutionary atonement as "vampire Christianity," wanting Jesus only for his blood.
What's missing from this narrow vision of salvation--salvation as avoiding punishment--is a positive view about what salvation means for our lives and how salvation is a developmental process. We "are saved" but we are also "being saved."
This is the classic contrast between justification and sanctification. Given its forensic framing, penal substitutionary atonement describes justification, being declared "innocent" and "righteous" in a legal context. Having been declared righteous, though, what's next? You walk out of the courtroom a free person and now have to face and live the rest of your life. For many Christians, this is an anxious prospect, as they feel they have to walk a very fragile, precarious path going forward in the face of the fear that they could "lose" their salvation. Others, having secured a get out of jail free card by the blood of Jesus, never turn to invest in becoming mature, wise, faithful, disciplined, and loving followers of Jesus. We've been justified, but we never get on with the pressing task of sanctification, conforming to the image of Jesus.
This is a problem. The moral witness of Christianity is abysmal. And much of this is due to reducing salvation to penal substitutionary atonement. People "get saved" all the time. Christians who have been "saved" are a dime a dozen. They're everywhere. But Christians who are mature, wise, faithful, disciplined, and loving followers of Jesus? Those are very rare.
Penal substitutionary atonement distorts the faith because it fails to teach us about what's next, how "the long obedience" of the Christian life is a process of "being saved." Penal substitutionary atonement distorts the faith because it isn't able to tell you this very important and oft-forgotten truth: You are not saved when you accept Jesus as your Lord and Savior. You've just taken the first step in being saved. Being saved is a process that will encompass your entire life.