If you have the kind of autoimmune issue severe enough that people are offering you biologics, the answer to "is it worth the risk?" is almost always yes, imo.
Unchecked inflammation is guaranteed to wreck your health, whereas treatments for that inflammation only have a slim chance of major side effects. For cancer risks specifically, it's worth considering that (1) you have a fighting chance to beat cancer, whereas an autoimmune condition can cause permanent and unfixable damage, and (2) autoimmune diseases are themselves associated with cancer risk. So for something like colitis, which is associated with bowel cancer, your overall odds of getting cancer are exponentially lower with treatment than without. (And less serious side effects usually go away when you stop the medication :P)
It's also worth noting that newer biologics are more like a sniper than a shotgun, so the immunosuppression is pretty targeted; still worth being careful, but it's not like the vulnerability that comes with, say, chemotherapy.