What bothered me the most was this one time when Dax was criticizing Earth from orbit and talking utter garbage about preferring purple oceans. This bothers me, since the only process I know of which makes water purple is infestation by an ungodly, maybe actually impossible amount of sulfur bacteria, whose waste products would probably kill you dead within a few dozen miles of any beach.
But maybe they're pretty from space, sure.
What about some kind of plant life, something more like coral, or plankton, just with a different "pigment" than chlorophyll?
If the oceans are matted with them, that's kind of problematic too.

Well, with the Empire State or Chrysler, they used, principally, the vertical space to beautify the buildings (both buildings taper slightly toward the top, but is that more of a reflection of construction necessities than pure aesthetic choice?). In the iconic Cardassian buildings, they use the horizontal space for that curved tail, which is likely to be far more expensive than putting on a cool art deco hat.Why pay to make things like the Empire State Building or the Chrysler Building so beautiful?
(I use those examples because I think Cardassian architecture is a sort of dark/industrial Deco.)
It's not really an objection to its plausibility, I'm just saying it's an odd decision. On the other hand, maybe land was cheap. There's no reason why the capital has to be an old city that developed organically. Given the unity of architecture, the Union might've built it out in a field somewhere, like Brasilia.