Along with what others have noted, there is also risk associated with insulin therapy from hypoglycemia (low blood sugars). Low blood sugars can be deadly for type I diabetics, killing as many as 1 in 10[1]. This is at least one of the factors leading to insulin being a tool of last resort, and in fact, insulin is prescribed to type II diabetics with especially strong insulin resistance.

http://care.diabetesjournals.org/content/35/9/1814