Yeah, I suspect that, by the time Busiek started to write the character, the rings had been retconned to such a high level as to be character-destroying. You can't have a story about the Mandarin kidnapping Stark and forcing him to invent part of a robot-ship, if the rings are so cosmically powerful that the Mandarin doesn't need robots or ships, being able to do much greater feats with a casual wave of his hand. Byrne broke The Mandarin, by retconning the rings to a level where the deviousness, savagery, and exploitativeness that defines him were rendered superfluous.
Busiek tried to fix this by, implicitly, having the rings be lost or abandoned once The Mandarin recovered from his amnesia curse. In their place he gave him rings that were merely remote-controls for his tech.