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!