One flaw is the Vulcans.
Back in the original Star Trek they were confident, logical people quite above petty emotions. Spock would endure one of McCoy's semi-racist rants with a cool look and then puncture the doctors ego with a witticism so sharp it could have been found on a page by Oscar Wilde. It wasn't that he didn't have a sense of humour, he just knew how to use it. Then, slowly, we met other Vulcans and could see just how unusual that was for them. many wouldn't even bother with the wit and just ignore the jibe. It showed Spock wasn't really the emotionless being he was supposed to be and gave him and the whole race a depth.
Over the years though that changed. No longer a coldly logical people the Vulcans became arrogant, even going so far as to sneer down on humanity. Just the odd Vulcan at first then more. Like an older brother laughing at the baby trying to tie his shoes for the first time.
Vulcans were once one of the most fascinating, well rounded and liked alien races out there. Now they just feel like a bunch of spoilt kids. Bullies that are as racist and cruel as the darkest days of the British empire, or the American 1960's. The very thing they were a rebellion against.
Turns out revolutions really do go full circle!
