When I was running a city adventure, one question constantly frustrated me: "What does this street look like?" Vornheim will generate the buildings on each side of the street, the layout, and an encounter every so often. That's all great, but it won't help me to figure out that crucial question. What kind of neighborhood is it? How many people are around? What are they doing? Can I steal from them?
All this comes into hyperfocus when a chase scene starts. Can I get onto the roof? Is anything blocking the PC's way? Is there anything they can throw in front of their pursuers? Is anyone in front going to try and stop them? Is there any way to escape from this alleyway?
When I try to think up this stuff on the fly, it just ends up bland and vague. It has the same problem as the D&D wilderness. The PC's spend most of their time groping through an amorphous cityscape with no detail or orientation. Every now and then they hit a sudden point of precise detail, like an encounter or dungeon. Outside of these pre-made bits the city has little personality.