Unrealistic? In a fantasty RPG that has magick, elves, goblins, werewolves & vampires just to name a few. You're concerned about realism? Right.
Anyway, floating cities are already a part of TES lore.
I'm gonna give lich freak the benefit of the doubt and assume they meant unrealistic within the world's rules (assuming they didn't know they existed already). After all, a floating city is a lot of rock, requiring a lot of power, assuming magicka is used it would require some seriously potent magickers to do it. So it is sort of unrealistic, except it's happened.
I wouldn't want to see millions and would be ambivalent about one or two. If they did exist, make them amazing, give them deep lore and story, make them a true spectacle. Otherwise, I don't really want them there.