There are a few things that can cause buffeting in a car. The most common cause is turbulence from the wind on the inside of the car and outside of the car. Another common cause is from the air pressure inside and outside of the car.
There are a few ways to reduce wind noise in a car cabin. One way is to install a window air conditioning unit. Another way is to install sound proofing materials such as foam or acoustic tile.
A car is most likely to be affected by buffeting if it is travelling in a high-speed lane and the windscreen is directly in front of the car.
There is no definitive answer to this question as it depends on the specific car and wind deflector. Some people may find them worth the investment while others may not. Ultimately, it all comes down to personal preference.