In general, labeling "restrictions" can't prevent anything. At best, they will shift responsibility.
Use against explicit instructions is considered Abnormal Use, provided that there is no reasonable way to prevent the unwanted use other than labeling. Manufacturer's are expected to address Misuse (Use Errors), not necessarily Abnormal Use.
The essence of the De-Novo process is reclassification (in fact, an attempt at down-classification - from the automatic class III designation to either class II or class I). So it's a matter of assessing the risk level inherent in the proposed device (including its labeling). If the FDA will conclude that the mitigation means are not as robust as they could be, it may result in a classification higher than the lowest potential. Not the best outcome for the submitter.