The problem with bump stocks, or bump firing in general even without a bump stock, is that it requires that the weapon be held in such way that it minimizes accuracy, even if you are familiar with the recoil of automatic fire. A bump stock is fitted to the rifle in such a way that the rifle is not fixed to the buttstock and pistol grip. You then pull the pistol grip and buttstock toward you with your primary hand, while simultaneously pushing the rest of the rifle away from you with your off hand. This reduces the amount of control you can exercise over the rifle so you are literally spraying bullets at nearly random. With a normal rifle, even one with full auto or burst mode, you are pulling the entire rifle against your shoulder, which gives you much greater control over the weapon.

A bump stock discards control over the weapon in favor of rate of fire.