Other than the obvious risks of containment failure, is there any reason we shouldn't use these for on-grid electricity generation? It seems safer than nuclear reactors to me, and provides almost constant power. I'm assuming they cost a lot?
Why yes! Yes there are. (1) They're about as inefficient a reactor structure as you could ask for. (2) They slowly turn from nuclear fuel to nuclear waste before your eyes. (3) The more power they put out the more radioactive they are. Compare and contrast - the RTGs on Voyagers 1 and 2 weigh 38 kilos and put out 157W. Good thing it's got three; that's 120 kilos of generator (13 kilos of plutonium - about 1/5th of the bomb we dropped on Hiroshima) for 460W. On the other hand, the solar panels on Juno weigh in at 340 kilos but in earth orbit they put out 12,000 watts. Tweaked to work out there around Jupiter they still put out about 500W.