I've known one or two of these mastiffs, and they are really big slobs with a sweet manner, and especially good with kids. You are right about that -- a shepherd can be more edgy. I would think the main deal is the cost of feeding, and then any medical issues.
If she has had trouble selling them, it would be good to find out if it's mainly the size and the feeding that puts people off, or something else that is harder to foresee. If she will tell you. I know you have a kind heart, but I hope she is not telling you she will have to bring them to the pound to push you into taking one.