It's not that Marriott....or anyone else....is incapable of error. "Even Betty Crocker burns a cake once in a while".
But my experience with Sheraton....and the sister Starwood brands....has been that it's really a roll of the dice. Some really great properties along with some real stinkers!
In 30-plus years of travel, I've tried 'em all. I find that Marriott-branded hotels across the board offer the most consistent levels of high quality and service.