There's a few things they can't lie about. They can't lie/make up any deals for lighter sentences or whatever. Note, that doesn't mean if they say 'we'll try to get them to easier on you' means that will happen, but if you (and your lawyer) actually strike a real deal (say to provide enough evidence to convict others), the police can't screw with that.

They also can't lie about the legality of actions while in the visible position of being the police. That means they CAN lie about it when undercover.