Having a short half life where it decays into something with a very long half life doesn't really eliminate the risk. Especially if you're dealing with forms that are water soluble. And it is quite possible for a reasonably strong alpha emitter to have a long half life. Alpha emitters aren't normally too bad, fairly low energy, but if you were to ingest a lot of one (like in your drinking water) there would be serious consequences for such a persistent long term exposure.
Plutonium dioxide has an 88 year half life, and is water insoluble. When RTGs are used, they're heavily encased and designed to survive catastrophic launch failures, and have done so, only to be collected and reused on later launches. If they had put an RTG on Philae, your chances of being harmed by it would have been approximately equal to your chances of being devoured by an Indomitus Rex. Unless you're planning to break into the sealed casing, slice off a chunk PuO2, and put in in a sandwich, you'll probably be okay.