The Sith ideology of taking over the galaxy is confusing and breaks logic. «Peace is a lie» is a phrase in the code of the Sith. First of all, if the Sith want to rule the galaxy, peace is not a lie, it exists as a shared goal with virtually most leaders, whether democratically or more authoritarian.
The Sith value the Darkness so much that they forget, the Dark Side is not the Sith. It is more chaotic than they ever will be. There is no control in the Darkness, but instead dangerous emotions and violence. The Sith do master the Darkness but it takes me to this. The Sith are so insistant that they have no limits but they actually do. They do not allow their Darkness to control them. That is a sign of restriction. When anyone falls to the dark side, they have this desire to to explore. And as showcased by Savage Oppress, they let their emotions enslave themselves. Now that is truely no restrictions.
To summarize it all, the Sith ideology is flawed because they hate peace, yet bring it through their Dictatorial regimes. They say they have no restrictions, yet do not allow their emotions to control themselves. The Sith value order yet the Dark Side is so much more chaotic. The Sith are more orderly than chaos and no peace truly is. and that is why they are flawed.