The biggest risk is hypoglycemia.
A decade ago some early studies convinced us that tight blood sugar control (e.g., glucose under 120 mg/dl or 6.7 mmol/l) lead to better outcomes in ICU patients, particularly in coronary bypass surgical cases. The American College of Physicians says 140 to 200 mg/dl is good enough (7.8 to 11.1 mmol/l). The article at MedPageToday didn’t mention open heart surgery specifically, however.
Science marches forward!
Diabetic Mediterranean Diet 
Excellent comments. I asked my Doctor about this and he also subscribes I assume to the idea that people heal faster in hospital and less die when blood sugar is 140 to 180.
He felt that super tight blood control for sick people in hospital was not helpful and more die