I think one of the difficulties in being a musical generalist who performs Bluegrass, Old-Time and Country Blues convincingly is that these styles tend to have very different rhythmic feels, and even within any one of the different styles there are a variety of different rhythmic feels, as in the way Earl Scruggs's smooth roll contrasts with the punchy, straight-up-and-down feel of Ralph Stanley or the bouncy feel of Allen Shelton.
I would make a distinction between performing repertoire from these various styles and playing in the styles. I have never thought of Doc Watson as a Bluegrass player, though he does make Bluegrass songs part of his repertoire from time to time. I just think Doc plays in his style, which encompasses Old-Time songs, Bluegrass songs, as well as occasional Country Blues, Country and Pop Standards. It's all just Doc.
Jody Stecher is a present-day musician who not only plays repertoire from Bluegrass, Old-Time and Country Blues, but can play in each of the styles convincingly, which is far rarer.
I'm dubious of musicians who think it's easy to play in a variety of styles. I think the best way to do that is to take songs from different styles and express them in your own style, as Doc has.