Awesome, I'll check out the site, though I don't have a lot of Nantucket folks in my line.
Here's the trick: Most probate records aren't indexed yet at FamilySearch, nor are many other items, so you can't use the search function to find them. But... in their catalog find the item, such as "Suffolk County, Mass > Probate Records." Use AmericanAncestors to find the file number, page, etc. Then look throught the item on FamSearch to find it. Also works well trying to find the original vital records. If, for example, it's listed in Bowman's VRs, he also lists the Vol. and page of the original. Find the original VRs in the FamSearch catalog, then page thru until you find it.
Slow, but rewarding when you see the true original record. I guess I'm an "old record" geek.