Who is not a good candidate for nissen fundoplication?

Depends: Initial considerations are a person's general health and risks for surgery and anesthesia. Other indications to avoid surgery would be gastric or esophageal muscle disorders (gastroparesis, achalasia), scleroderma, and those whose symptoms are not from reflux disease.

A Different Spin...: In addition to dr. Hoepfner's excellent answer, i would argue that anyone who can control their symptoms nonsurgically (diet, meds, lifestyle), is a "bad" candidate for surgery. Lap fundoplication is major surgery thru minor incisions, and forever changes the way one processes food--it is not for the "casual refluxer".

Severe dysphagia.: Mild dysphagia can have a partial wrap. Prior surgery makes the case difficult so a procedure such as a tiff may be better. Prior gastric or esophageal resections are problematic.

When it won't work: If the operation won't help the symptoms or meet the patients goals, it shouldn't be done. While this cannot be predicted with 100% accuracy, a careful history along with tests of the upper GI tract can maximize the chmaces for success. It should not be done in patients with achalasia. Gastroparesis that is mild and has predominantly gerd symptoms may benefit from nissen. Severe gastroparesis not.