If I had any qualms at all - it would be the "group" vocals. They seem washed out a bit. My guess is that they were recorded separately, and that one or more vocalists' low-mid range is muddying up the spectrum. Try to re-create the "old" way of doing bluegrass vox (all at one time around a ribbon mic) by leaving frequency room in each vocal track for the other vocal tracks. (I hope that makes sense). If, by chance, these vocals were recorded live around one mic, I would have chosen a different mic.
Having said that - the song sounds good. Your every day bluegrass listener would never hear anything 'wrong' with it whatsoever. But, you did ask for some critique, and that's was the only thing I could think of - and I wouldn't lose any sleep over it.