Frank might indeed have turned out to be as callous and dangerous as Willoughby, and might have wound up hurting his father and Mrs Weston, as well as Jane and Emma and the people who love them.

Fortunately it wasn't so.

However, even with Frank as a relatively benign character, I think his scheme to throw people off the scent by appearing to court Emma could have been extremely hurtful for her. She might so easily have fallen head over heels in love with him, and the discovery that she had been the victim of a cold-blooded plot would have made things doubly painful. She would not even have had the consolation of knowing that she was the one who was really preferred by her 'Willoughby'.