The majority of riders uses a bit, although they do not know the use and how to use it. If used improperly or when used a wrong bit the damage can indeed be severe. That is why some bits may cause a protest reaction of the horse. Without reason it is then often caused on the horse.
There is a huge offer of different types of bits. This doesn't make it more easy to pick one. Every rider and horse are different, that is why not every bit will work for every rider or horse. A bit is, on it's own, never the cause of much damage. It is however the way it is used. Even more the bit needs to fit the horse, if not, you can be the best rider in the world, sooner or later it will cause the fatal tension on the horses mouth.
One advice, take your time to choose the right bit. Let yourself being advised by someone who knows the different types and uses. And never forget to closely watch and listen to your horse.