"But that has never stopped Mudcatters from smearing a whole genre in the past.
It's also possible that the poster in question has a low tolerance for sentimental songs."

It isn't just the sentimental drivel but the propagandistic effects that it has which is used by the Southern White Ring. The pro Southern side of the Civil War is trumped up, and the "South shall rise againsters" belie the honesty of the lyrics.

As to the nature of Bluegrass, it is a Johnny-Come-Lately as of the 1940's.
It isn't fully matured as a genre of music as is say jazz. It is not above criticism as an art form.

As to sentimental songs in the 19th century, they were a mixed bag. Some were jingoistic such as "If you don't like your Uncle Sammy" to forerunners of the German popular song sentimentality that predated the 1920's in Berlin.

It's not the sentimentality I object to per se but the use of it as a racist propaganda tool as was done with the song "Dixie".