It does feel like a knock-off of Sigil.
Within the entire, established D&D cosmology and multiverse, if there was a demiplane city that was populated by hundreds, or thousands of 20th+ level characters, where even the petty shopkeepers were 15th level Experts and Adepts, the poor beggars are 14th level commoners, and the typical city guard is powerful enough to defeat entire armies on the battlefield, where there is an assassins guild with poisons that can kill you so bad that True Ressurection and Wish can't bring you back (one bit of hooey from the ELH I thought was just plain stupid), all meant to be a hub of commerce and adventuring on the planes, I'd have thought it would have been mentioned by now.
It all sounds so over-the-top, like one too many expansions in an MMORPG as they try to come up with new places for ever high level characters to go and fit in.
Everything Union was supposed to do could be filled by Sigil: Hub of planar commerce, society and travel, meeting point for heroes from across myriad worlds, surreal city that could have countless adventures unto itself, and place where high level or epic level characters could use their abilities but not utterly overpower and ruin the town.
We already have Sigil, The City of Brass, The City of Glass, The Sixteen Gate Towns, & Dis. I could even accept a demiplane city named Union run by the Arcane (now Mercane) as a part of it (and I did throw it in to my last campaign as just another planar city, albeit a wealthy one), but not as The Epic! City.
I am of the mind that 20th+ level characters should be somewhat rare, powerful and influential. Major planar metropolises may only have a handful (and with that in mind, the major players in my version of Sigil tend to be in the 20th to 30th range in level). Entire adventuring parties of Epic characters are very rare, and known ones should be able to be counted on one hand (going back to the MMORPG analogy, like uber-guilds on a server)