How likely is dying from open heart surgery?

Heart surgery. It depends on the type of heart surgery, the condition of the patient and the skill/results of the surgical team and support staff. In really first rate places the overall mortality across procedures can be 1% or less, tho some will have much higher and some almost none.
Some risk but low. Usually it is less risky than not having it if you need it. The risks vary with what is to be done congenital disease coronary disease valvular disease risk rises with age, obesity, diabetes poor heart function and some othe factors.