This question was prompted by discussions on this question. Can there be an advantage to using a molecular propellant in an ion engine, as opposed to noble gasses or iodine? (Although iodine actually does look pretty cool.) There's a large variety of molecules to choose from, but it seems like you could find some that are cheap, heavy, easy to ionize, and store well.
I can think of a disadvantage, which is dissociation. Like the quench gas in a GM tube, except quench gasses are specifically chosen for that property.