I tried a Thorn flat track bar to try on my old bike before getting a new Raven Tour.
On long rides I get hand pain with flat mountain-bike bars (not just Thorn flat track, any simple fairly straight handlebar).
This could be addressed to some extent by using bar ends, but I finally decided to have drops, as I use the position with my hands on the hoods a lot of the time on long distance rides. With drops and repeater levers I can brake from all 3 hand positions, I can't do this if using bar ends on straight bars.
With increasing age I expect I will eventually go to North Road type bars rather than typical mountain bike bars if I can no longer use drops comfortably.
My Thorn flat track bar is now on my utility bike, which gets used for local rides.
As far as I can see the main difference between Thorn narrow and Thorn flat track is the angle - narrow is 5 degrees, flat track is 10 degrees. Both bars have graduations to easily trim the width