The Seven Sisters aren't representative of ugly mass of concrete Soviet-era architecture though. I'm not sure I'd call them beautiful but they're rather distinctive and they don't jump out as particularly ugly. Wikipedia claims they're a combination of Russian Baroque and Gothic styles but someone with more architectural background than I may have a better classification.
As the Wikipedia article also notes the Seven Sisters have considerable similarities to icons like the Wrigley Building in Chicago. (Not sure I buy a couple of the other comparisons.)
But Russians sometimes call the "ugly mass of concrete Soviet-era architecture" French style... And well, as a French, I can't claim they are wrong about that :-/
Yes, that accounts for one part, the one with ideas behind, a style, a urban and a life project. And then there is the other part, the loosely inspired, derived copies with no real plan.
Not sure which part brought the worst failures :-)