The limitations to using steel (of whatever size) to represent paper targets are: 1) it encourages a pass/fail approach to shooting paper, because all hits on steel are the same - not true for paper targets; and 2) auditory feedback extends the distance at which that "shot calling or act of faith" dynamic kicks in (to essentially infinity), especially if you're not combining the shot with something else that would reward you for calling the shot (such as leaving the position or taking the gun to another target). If you're mindful of these things and take steps to control the potential consequences, they shouldn't be too big a deal. But just going out and whaling on a steel IPSC silhouette of whatever size can train some not-optimal habits.