I did some research on the Big Tunas a couple of years ago, and was told that they are grafted onto Scots Pine rootstock. However, I personally didn't see them doing it, so take that little bit of information with a sizeable grain of salt.
I have one kicking around here that seems to be happy being a little tree... and, over time, I think the bark variations will blend out so that the graft is barely noticeable. If I was going to guide someone on buying one of these, I'd just say that it is all about the skill and luck that the grafter has or had at the time. Since the graft is never done with bonsai in mind, they can be somewhat ugly when completed.
I've been letting mine recover for a couple of years now, so this season I'll begin to give it a little work and see how it reacts to it.