The thought of sweet potatoes makes me urge. A couple years ago, I had just gotten into eating them and had them one evening and a couple hours later the stomach flu hit. Well, you can guess the rest I have not been able to try them since.
I would think that boiling sweet potatoes would extract some of the potassium and nutrients from them. I could be wrong, but I stick with baking them in the over for an hour @ 475 degrees.