What is the determining factor for ordering maintenance BCG? That sounds like a question for your urologist or oncologist. I am not a human doctor and can only relate our experiences as a patient or caregiver.
As to side effects, all my husband experienced was a brief 1-2 day period of bladder irritation right after the instillation. On this forum I have seen others complain of UTIs, blood in urine, bladder pain, and systemic effects like fever, muscle aches and cough. Because it works as an immunotherapy there is the potential for other side effects like with other immunotherapies. You can research this online or ask your doctor about the potential for side effects. It is always good to know what to expect when starting a new treatment.