I think the answer will depend on the health condition in question and whether existing journals have featured the exact condition/ treatment in other forms of methodology, such as randomised trials. However, even if RCTs on the same topic have been published, an unusal case or case series may bring valuable new information, or be a nugget of information for rare diseases or new treatments. As a former editorial member, I would probably be more concerned about whether there was a good definition of cases, treatment exposure, patient demographics and that any case series was consecutively recruited, if possible. It's worth looking at previous articles published in journals and perhaps an initial enquiry to journals to see if they would consider you paper in principle.