STACK looks very promising, I think it could do a lot of my question types, probably a lot easier than I can do them with WebWork. (To balance equations, I had to manually parse the equations, count the 'atoms', setup linear systems of equations, and solve them using matrices - which is fun, but definitely non-trivial).
A common approach I have in my questions is a list of items with a match (e.g. 'H' => 'Hydrogen', 'He'=>'Helium', etc.) and then choosing N items from my list and displaying one side or the other. I suspect that Moodle can probably do something similar, which would make transitioning reasonably easy.
I definitely can embrace switching from WebWork - its power comes at a cost of complexity and a rather loose coupling between WebWork/Moodle.