I think it depends on how unusual the window shapes are. If they are primarily rectangular, I suspect that once you have a library built up that you'll be fine with Revit. But there is no analog in the 'standard' Revit windows to being able to add/delete mullions from a simple dialog in the style.
It is possible to build a revit family that has the option to add/delete mullions dynamically, but building them is going take a little time. The attached pdf has a procedure for building a custom door that includes a lot of the steps for something like that (and a lot of other advanced stuff). It's not simple, but I suppose after you build a few, it won't seem so hard.
On the other hand, really odd windows may be eaiser to make in Revit that ADT, because you do build them from scratch, and ADT does not work too well when you need a odd type it didn't include.