Lying about witnesses accusing you is bad enough. But they’re well known to blatantly lie about things like what the law actually is, or sentencing or how good a deal you can get for confessing.
The worst part is the final deal is up to the judge, usually following the recommendations of the prosecution- not cops.
Most of their games are curtailed significantly even having an incredibly shitty public defender. (Don’t mean to rag on them. They’re fighting the good fight. But they’re over worked and spread too thin. A public defender is never going to compare to a private attorney- never mind an entire legal team. Just saying even the absolute worst legal aid you can think of is going to stop most of it.)