Are you saying that underhanded FT shooting is inherently more accurate? My pops "pitched" 99 straight in HS underhanded and taught me the same but I converted to traditional and can still drop 88 to 93 of a hundred most days.
Personally, I think it is easier to create a consistent one handed action over pitching them underhand. That is just my opinion of course but I still teach FTs the same way my coach, not my pops, taught me....but I have been changing the release and guide finger over the last decade based on new concepts that seem to improve accuracy.