Quite correct and shielding necessary to conceal and safely transport such a device would make it prohibitively heavy and large.
Much better bang for your buck with conventional explosives. Don't get me wrong it will make a big mess but that is about it.
The shielding would depend on the radiation involved. If the isotope’s decay does not have much in the gamma, but more in the alpha and beta, the shielding requirement is comparatively reduced. This is especially the case for an alpha emitter, which can be shield with Saran Wrap. Ingesting alpha emitting material though is quite nasty. An example is the polonium tea served to a Russian dissident.
[Much better bang for your buck with conventional explosives. Don’t get me wrong it will make a big mess but that is about it.]
If such a device were detonated in New York’s financial district—New York Stock Exchange, Federal Reserve Bank of New York, Wall Street, etc.—it could cause a big, negative ripple before the site was cleaned up and confidence restored. The weapon could be delivered in a large truck.