Most bracket-mounted bumper guards are somewhat wider and taller than a standard license frame. The depth or thickness of a plate holder made to protect bumpers, on the other hand, is quite a bit more. Some frames have depths as much as two inches or more of thick rubber material that compresses on impact and then returns to its original shape.
You want a rubber license plate holder to be able to absorb shock and then return to its original shape to protect against the next trauma. Ideally you want the material to be strong, but also flexible, even in freezing winter weather.
You want a bumper guard to be made from material that is not only shock-absorbing, but also weatherproof and able to withstand rain, snow, or any other weather that is thrown at it.
Metal license plate brackets are notorious for losing their luster, bending, and corroding away. A rubber license plate holder will replace your frame, and if made from the right materials will never rust or bend out of shape.
Most rubber holders are made from foam rubber and may experience sun damage. Ultraviolet light from the sign is not friend of foam rubber or plastic. Make sure any holder you buy can withstand the unrelenting sun day after day and year after year without compromising its capability.
Every vehicle is different. Most come with at least some mounting hardware, but some do expect you to use existing license plate mounting screws. The problem is that plate holder bumper guards are thick and the existing screws may not be long enough.
Also, keep in mind that most manufacturers are expecting you to attach their bumper guard with four screws. Although most license plates and brackets can accommodate four screws, the way they are attached may only use the top two. If your license plate and bracket are only attached with the top two screws, you will need to adjust the attachment so that the bumper guard can use all four screw attachments.
Often when a taller car backs into your car, they will ride over your license plate bracket and bumper causing possibly significant scratches and other damage. Some models of rubber license plate holders have a protective rubber extension above the holder that can help prevent at least some of that damage.
Even when someone else backs into you, you are at risk of liability. In the absence of video or photographic evidence, an unscrupulous motorist could blame damage to their car on you. The most common damage is when they impact your car in the front license plate area and the bracket and screws causes scratching, dents, or even holes in their rear bumper.
To protect against this, most rubber bumper guards have screw protection that is an extra layer of protective material around the screw area to keep the screws from impacting and damaging other vehicle bumpers.