Some of them require low health or high crime or super low happiness, stuff that would pretty much destroy your city for a long time before possibly slowly getting better. And they don't stay unlocked between cities. So how the hell is anyone supposed to achieve those?
I went after the Posh Mall, which required 4% tax rate across everything, for 20 straight weeks. I was raking in the cash before I attempted it. By the end of the 20th week, I had to taken out three loans and hit bankruptcy. Almost killed my ciy.