A constitution without separation of powers with the possibility of judging the representatives is worthless.

The fact that doing so is difficult (the most blatant case being Nixon) does not make it worthless, it only helps granting the executive some stability (because it has usually been elected by the people, so it needs some berth to operate).

In that sense, the US constitution is a very good early example.

The independence of the judiciary is essential.

I am not defending the US constitution per se, though.