I?ve read two books just about D-day and there was really almost no chance of failure.
Even if the Germans had reacted in the entirely correct manor they still would have never been able to force the allies into the sea.
The allies had too much fire power, men and air cover.
Of course it was still a HUGE gamble, and had it failed it would have added another year to the war, worse the Russians might have taken ALL of Germany before we could get on the mainland.