I've been guilty of this misconception myself at times...
"One of my many weaknesses is that I don’t, at least in my heart, believe that missionaries have weaknesses. I see them as super-heroes. How wonderful they must be to leave the comforts of home and family to go and serve. My mind knows better, but the heart has its reasons. Missionaries, I know objectively, are not super-spiritual people who do not sin. They are instead super-spiritual people whose consciousness of their own sin fuels gospel gratitude which in turn leads to sacrificial love for others."
R.C. Sproul Jr.