How dangerous is a megacurie (MCi)?
There are a lot of factors at play here:


The curie and becquerel only tell you the number of disintegrations per unit time—they convey no information of the type or energy of radiation produced.
How dangerous a radioactive source is depends on the type of radiation produced and how one is exposed to it. External exposure to alpha particles is not very dangerous because alpha particles are easily stopped by skin, but inhaling alpha emitters can be very dangerous because there is direct access to sensitive alveolar cells.
How the radioisotopes are dispersed will greatly affect individual exposure to the radiation. A large amount of radioisotopes spread over a large area may increase the incidence of cancer slightly for a large number of people, but a smaller amount concentrated in one place may be very rapidly fatal to a single person.


20 MCi (740 PBq) is indeed a very large amount of radiation (~1/7 that released by the Chernobyl accident), but in a big cloud spread over thousands of square kilometres may not be acutely dangerous to people. That many radioisotopes is equivalent to 231,000 kg of cesium-137 and all that (magically) concentrated into a point source would produce a dose of .62 Gy/s for a person 1 m away—fatal in seconds.