A client reported that their employees were able to enter invalid names/labels during template creation which resulted in pods not starting. A suggestion to validate them on creation in order to avoid this issue.
This validation introduced the constraint that PodTemplateStepExecution.label contain only a single value, rather than a label set. In this context (as opposed to KubernetesCloud configuration) that is probably fine, though it means that the GUI label Labels was misleading.