Well, one bad thing is that existing cities would need to be re-designed. It will take resources and decades of time. If we are patient, there is nothing bad there.

A potentially bad side effect: if planners take the easiest route and make the city ultra-dense and ultra-high, we get a vulnerable city that doesn't function if something is wrong with the infrastructure. People are, after all, known for taking easy routes (which may later prove hard for others).

Myself, I live in the countryside and don't like top-down planning at all, so I can't comment more. To me, the experience is typically: "can I build a road here? - no you can't", "can I make a thermal store? no you can't", I'm sure they will eventually tell me I'm producing solar energy the wrong way too... I know that things are different in cities because the threshold to disturbing others is tiny, but I don't think about it much - it is not for me.