I've owned a TC Thunderhawk in 54 cal since the year they came out. So I can hopefully give you some helpful info.
It's a TC- so fit, finish, overall quality is above average.
I have the blued/wood model. 2 partners each have the stainless/synthetic model- less outside maintenance on those.
Mine's always been a decent shooter with heavier conicals. Never could find a sabot/load that I liked. Barrel is kinda short- 20 inches. Always thought accuracy might be better with a longer barrel
Sights were not so good- replaced with rear peep and fiber optic front. Sights now may be hard to find since the gun has been out of prod. for awhile and I don't think was ever real popular.
Gun is nice and light. Recoil is noticed with heavier loads.
It does not have a removable breech plug- one of the biggest detractors- and possibly why they didn't make them very long.
Came with good triggers. Think its a Timney.
Summary: Decent little inexpensive gun for the quality. But if I was serious about BP hunting and wanted only 1 gun a Thunderhawk would probably not be it. My

p.s.- They are legal in WA State as is. The TC Firehawk and then Black Diamond I believe replaced the Thunderhawk. I have a Black Diamond and the actions are very similar; a few refinements and a removable breech plug were added.