Also a bizarre comparison. Cities 2 is a simulation game - they are very CPU intense games. The graphics are nice but it's likely it's problems with balancing the CPU demand and the graphics that is the problem, rather than the graphics themselves. Simulation bottlenecks will drop the FPS drastically, regardless of the graphics engine.
From what I'e seen of the game on Twitch, I think the performance issues aren't game breaking. It seems the game runs fine if you reduce settings; while it's far from ideal it looks playable.
But it will be damaging for the game. Mods won't launch until after the game is launched, and that may be delayed further by time taken fixing the game post launch. For a game that suceeded in a very large part due to user content that may really harm the game's success.