To confirm a state, the comparison should use a model whose relevant property is already accepted by the audience, thereby reinforcing the proposition about the subject. A disputed model would offer weak confirmation.
For embellishment, the image should make the praised quality attractive; for disparagement, it should effectively disclose the negative aspect. One model cannot be presumed suitable for both goals in every context.
Thus the distinction between acceptable and unacceptable goes beyond verbal form to involve audience, context, and rhetorical purpose, integrating the previously studied purposes of simile.