Shelties are notorious for bad teeth. We brush their teeth and the vet always says they look great. Well, suddenly, one of Eli's lower incisors literally flew out of his head while we were playing frisbee. And when I looked, the one next to it was loose. Off to the vet. She said the loose one needed to come out and she was concerned about at least one more. She took Xrays which showed more teeth were marginally hanging on because the bone in his jaw had thinned. So he lost a few teeth, mostly the small premolars, but she said some others were marginal as well.
Her comment was "Well, genetics won this fight". He just turned 9.
The problem is not the tartar or plaque, it's whether there's any gum inflammation. That bacteria can seed itself in their kidneys and other organs.