Actually a weak point on most armored cars is the door seams. At close range a high powered weapon aimed at the door seams can go right into the passengers compartment. As you said the element of surprise is critical to the operation of these vehicles. Part of any drivers training is to get the car moving ASAP if they are being fired on. That makes those critical shots nearly impossible even if your right up on the vehicle.

Many of the custom armored cars used by governments are destroyed after their useful service life to prevent their design flaws from being known to the general public.

I find them somewhat interesting but they become quite impractical very quickly. Its almost like driving a moving van. Braking and handling are very much impared. Even the big v8's have a diesel like feel because of the weight.

The S class is the most popular choice for diplomatic cars because it handles well before the conversions and has several strong engine choices. Some cars become quite unsafe with the extra weight especially in a situation where the car needs to escape a situation. Its no good to get your VIP away from an assasination attempt only to kill them "lady di" style in an accident.