This is true, and the difficulty in firing bad cops is a testament to the power of a well organized union.
But I generally think far too much emphasis for the problem of police unaccountability is placed at the feet of the police unions. Unions often make it difficult to fire petty offenders on the force and that can lead to a culture of corruption and malfeasance, but the problem of police using excessive force is, first and foremost, a criminal matter that supersedes any union contract.
The fact that examples of criminal behavior is often treated as simply a HR dispute is a big part of the problem of our unaccountable police. Excessive force is not just breaking policy that should be fireable, but often a criminal violation of civil rights that can and should be prosecuted. The fact that many DAs are extremely reluctant to actually do their duty and prosecute criminal cops is a much larger issue than any iron-clad union agreement.
I don't mean to strawman you here Joe, your comment merely reminded me of this gripe.