What are the advantages of immunotherapy over chemotherapy?

None. We all wish that we could learn how to manipulate the immune system to battle cancer without chemotherapy but this has been an empty promise so far. There is a lot of ongoing research in immunotherapy but to date they don't compare at all with chemotherapy. Some cancers may be more amenable to immunotherapy and there have been some promising results, but we are still a long way away.
Theoretical. Theoretically immune-based therapy such as vaccines may have advantages of "conventional" cytotoxic chemotherapies. Until recently, most immunotherapies were not very effective; however, there are now therapies for melanoma & prostate cancer and active research on other cancers. Not all "chemo" is extremely toxic & some immune therapy is (eg, il-2). Advances in all ca tx's are promising.