There are additives that one can add to the blend to counter that effect, but it's very limited, how much you can do. You can improve it by one or two degrees. That's basically the effect of additives.
That's the fundamental issue on the negative side.
The other one, quite honestly, is that the sources of biodiesel are so varied. I mean, they can come from french-fry grease, grains like canola and soy, or animal renderings, and when you turn those into biodiesel, the properties are quite a bit different.