Tire blowout, mechanical failure, deer running directly through your direction of travel, a vehicle pulling out of a dirt road you didn't see, and didn't know existed. Glad you asked.
How would cruise control slow down one's reaction time to either, given that they use a safe following distance and pay attention to signs indicating that on-ramps are coming up?
I think you have your answers.
"Tire blowout"
I use decent tires, keep them properly inflated, and replace them before they're worn out. That seems to have kept the incidence of tire blowout on my cars to zero.
"mechanical failure"
I'm well aware of the mechanical condition of my vehicles, and I keep them in good repair. That also seems to greatly reduce the incidence of mechanical failure.
"deer running directly through your direction of travel"
I'll give you this one, with the exception that most of the deer accidents I know of involved the deer running right into the side of the vehicle. The driver never saw the deer until it was in the side window.
"a vehicle pulling out of a dirt road you didn't see, and didn't know existed."
Limited access highways, by definition, do not have "dirt roads you didn't see and didn't know existed".