It really isn't. It's not an optimized port, but it's certainly not shit. It's fully playable and most normal people will be more than happy with it.
I've almost 100%ed it on Wii U. Its got some minor slowdown in larger cities but you almost never notice it. It doesn't have any screen tearing and the lighting's quite gorgeous. It does have some aliasing issues but overall its very playable. I've talked to friends that have the same version and they all agree. It seems like the only ones shitting on the port are DF and people who don't own the port themselves.