Ardana is an example for a Federation member with a rather tyrannical government:
Then again, depiction of the Federation in TOS significantly differed from the one in TNG/DS9/VOY. In TOS, the Federation was the United Nations... in space (with Earth acting as the United States in space)! From TNG onwards, the Federation as a whole
was depicted as the United States... in space!
It's easily explainable in-universe though, because it isn't too far-fetched too asume that the nature and structure of the Federation had evolved over the centuries. (Again comparable to the evolution of the United States: In the very beginning, the U.S. was a merely a loose confederation of states. Sometime later, it was an actual nation-state, but one that allowed slavery in it constituent states. Nowadays, it has a black president.)
That being said, it's hard to imagine that the 24th century
Federation had allowed Ardana to become a member planet.