I think the word nuclear fools people. Think of it as a... I don't know... yogurt bomb instead. Wrapped around conventional explosives, the yogurt splatters when it is detonated. People nearby get a whole bunch on them, which is bad. But it can be washed off somewhat, but look for these folks to have high rates of cancer cases coming up.
Instant deaths will not be the big concern with this type of weapon. Contamination of buildings and water will however. "Only" dozens or hundreds of people may actually die in this attack originally, but a certain radius around the impact point will most likely have to be abandoned for years and years until the radiation mess can be cleaned up.
If the bomb is large enough and placed say next to the capital, our government will have to move out of D.C. for many years. If it's placed in an urban section of NY, LA, Chicago it would be devastating financially more than in casualty numbers. Think of the impact WTC has had on our economy, then picture it if every office building in a 3 mile radius of WTC had to be condemned for 5-10 years.
So what's the downside of this bomb?