They can quickly develop into a life-threatening condition called ketoacidosis, and treatment is very expensive (24hr emergency care for several days). Unregulated diabetics are more prone to this.
I know this from experience
My mom was on a med that caused her to develop diabetes without her realizing it. Undiagnosed and untreated she developed ketoacidosis and was in a diabetic coma for about 2 weeks. It was very scary.