There are some real unintended downsides to multiple use zoning. First off, unless the zoning is extremely finely defined, you get situations where people are expected to live across the street from a towing company that enters/leaves its yard all night long with its lights flashing and its loud engines and brakes doing their thing. Then you have the bars/taverns who turn their customers out into the neighborhood at 2:00 AM to loudly wake the sleeping residents (multiple use zoning almost always restricts the size of the parking lots, which disperses the cars onto the streets where people's homes are.)

And let's not forget those famous pizza delivery guys. Their story involves the fact that pizza delivery guys drive by each other many times each night because people don't really care which pizzeria is the closest, they want the pizza from their favorite place. Same deal with having shopping centers in every neighborhood. People don't necessarily shop at the closest grocery store (or hair salon or gymnasium...). They go to the one they like the best. Putting them all next to housing just puts more cars onto residential streets, in my experience.

Oh, and those multiple use zoning areas generally involve much higher density living than single zoning areas do. That can be great when your many neighbors behave well and don't have huge impacts on each other. However, I live in a relatively uncivilized city. It's not working here. If many people insist on driving many trips per day, creating smoke (tobacco, barbecues, fireplaces, diesel engines), playing loud music or whatever, it really does matter how many of them live around you. I have seen some studies indicating the constant noise that is endemic to European cities has been documented to take a measurable toll on the health of the people who live there.

These problems aren't unsolveable, but they are rarely mentioned by fans of the "New Urbanism", "Mixed Use Center", "Densification" or whatever name this is going by today.