For the first 8 or 10 years I hunted with a Muzzleloader, I used an old .45 caliber hawken styled rifle that belonged to my mother. For the first few years I used round ball and I took multiple deer with it, between 10 and about 60ish yards.
I recovered one bullet that had gone in one side, through both lungs, and lodged in the rib cage of the far side. I wish I could find it to post a picture of it, but it is in a box somewhere out in the garage. It had almost completely flattened out. It looked more like a lead nickel than a round ball. On one side the stitching of the patch was clearly visible, imprinted in the lead.
So, while I can't say how effective they'd be at longer distances, I can tell you that on a good broadside shot at 60 yards and less they will take whitetail.