There are two separate cases:
1) if PEMS fails, reasonably direct and serious harm can occur. For example, PEMS might be used to position the dental chair, and the failure of the PEMS could result in a serious mechanical injury.
2) if PEMS fails, and only minor harm, or serious but indirect harm (i.e. harm from a longer sequence of low probability events, with the PEMS failure just being one of those events). Example of minor harm might be wrong drill speed or curing lamp time is too short, so treatment is not effective but no immediately serious outcome. Example of serious but indirect would be if the failure of treatment has a small chance of eventually developing into a more serious problem.
In the case of (1), there really is no doubt that Clause 14 for PEMS should be applied, at least for those parts that provide protection against the hazardous situation.
In the case of (2), you could argue either way because the definition of essential performance is weak, but as I understand, the intent of the standard is really for higher risk situations as for case (1).
You can anyhow exclude Clause 14 if justified in the RMF. It's just that case (1) an attempt to exclude is obviously wrong.