What is the possible advantage that immunotherapy might have over chemo?

Toxicity/duration. While the answer varies, there are some general differences: 1. Chemotherapy typically causes a quicker tumor response 2. Immunotherapy is more likely to induce a longterm or "durable" response 3. Chemotherapy side effects are usually related to injury or death of non-cancerous cells, 4. Immunotherapy side effects are from inflammation of the immune system.
More info needed. Immunotherapy is most frequently referred to the treatment to induce tolerance to susbtances that produce allergic symptoms. Many new drugs with effects in the immune system are being called immunotherapy. Chemo refers to drugs affecting cell metabolism and cause their destruction, usually applied to cancer. Chemotherapy needs be compared to newer drugs for each specific condition to be treated.