IMO the weak spot is that there's only one executive branch, and they have discretion over whether to enforce the law in any given case. They can choose not to

IMO the weak spot is that there's only one executive branch, and they have discretion over whether to enforce the law in any given case. They can choose not to arrest themselves, or prosecute themselves. If the system were structured with two executive branches, each of which had the power to enforce the law against members of the opposition executive branch, and a sense of opposition / competition could be maintained, then this wouldn't be an issue. There's also the issue that the executive has all the real power. The courts and congress can send strongly worded letters, the executive has all the guns. This hasn't yet become a direct problem in the US, but it has in many other countries.