Much of the advice here is good, but some is a bit weird. > I will give the document to my brother, Tom. (The writer has only one brother. The brother's name is nonessential and therefore set off with a comma.) > I will give the document to my brother Tom. (The writer has more than one brother. In this case, the specific brother—Tom—is essential information and should not be set off with a comma.) I know this used to…
"Your work has been frankly awful" has frankly as an adverb. That is, it was awful in a frank way.
"Your work has been, frankly, awful" makes it clear that frankly is a parenthetical remark that isn't part of the sentence.
With commas 'frankly' modifies how the writer intends the sentence to be received. Without, it modifies how awful the writer things your work has been. I suppose in some situations, the absurdity of 'being awful in a frank way' is sufficient to resolve the ambiguity.