Are there truly any Major Sins in Islam besides Shirk? Is Shariah truthful on this?

If #Shariah is based off the #Qur'an and the #Sunnah shouldn't they all be in agreement on what the three major sins are? Reliance of the Traveller, aka Umdat As Salik is the source for Shariah we are using. If these sources disagree does that not make major problems for Islam? What happens when we apply simple #Criticalthinking to shariah?