Vall,
I don't think I can agree with that assessment of the Hepburn. It is strong. But that's all. So are any of the Sharps models, the 1885 Winchester and even rolling blocks. The side lever may be convenient, but they aren't great for dealing with stuck cases which are a fact of life sooner or later. Nor do they chamber a recalcitrant cartridge.
I don't see any advantage to opening and closing "... without changing his hold". After all, he will have to extract and then thumb in a cartridge with one hand or the other. Typically, the right hand does this for all right handed Hepburn shooters I know, and that's a fair number. I do not see a single ergonomic advantage to them among all the rifles on the BPCR and BPTR firing lines.
They are a fine, if odd looking and fitting, rifle. But I don't think they have anything on a Model 1885 Singleshot - which is also a heck of a lot better looking in my opinion.
When it comes to single shot hammer rifles, I don't think the American guns take a backseat to anyone. I just prefer the Winchester over all the others. Good as they are.