There's nothing wrong with Union in its basic concept. It's not very much like Sigil, and that's a good thing - it fills an entirely different niche. The mercane, for whatever reason, are terrified of Sigil, so it makes sense they'd construct a planar metropolis of their own. And Union's an interesting one.
But parts of it are extremely silly, to the point of damaging the game by ruining the suspension of disbelief. Making the average;/i] guard 15th level is just a stupid waste of even the mercanes' copious funds. The average inhabitant of Union, unless they have a spell that detects low-level characters at the gate and refuses to allow them to come in and spend their money until they've gained a dozen or so levels in a PC class, is going to be pretty as low-level as in any other city. Most people are low-level in general, after all, and if Sigil doesn't need every guard to be in the mid-teens in a city where powerful fiends run amuck, Union certainly doesn't. They should hire ordinary low-level guards to deal with the ordinary mugs and keep a high-level strike team for emergencies, hiring adventurers passing through for very difficult tasks.
That would really transform the city for me, making it a lot more appealing, to simply acknowledge that most people everywhere are low-level rather than to ruin the concept of being high-level by rendering it meaningless in context, in a city where everyone is impossibly, improbably, stupidly high-level, making high-level mean nothing at all.