I have figures from Black Scorpion, Dead Man's Hand and Knuckleduster and use them all in the same games. I've never had a problem or had someone complain about scale differences with these figures. I also regularly mix them with new Foundry and Reaper figures as well, so don't forget those. Artizan works too, but there is a bit more variance to the figure size. The real problems are old Foundry, Dixon, and the Warlord comanches (especially these last ones).
As for quality of the sculpts, they are very different. I'll second Lee494 that Knuckleduster makes very high quality sculpts and they are getting better all the time (especially since the Kickstarter) with more character and more dynamism. I used to think they lacked a certain eclectic look to them that I like, but recent sculpts have been getting really good. I like my sculpts to be as unique and character filled as possible (see the Madam, some of the soiled doves, the upcoming Judge or the gambler sculpts to see how good they are getting). One other note, is that Knuckleduster has many renowned movie characters from older westerns (think the Duke, Clint, and many more).
A lot of people really like Black Scorpion miniatures, but I find that I like them a little less as I've painted my way through the collection. The good is that they all have very active stances and look rugged (often very masculine and stereotypically western) and cinematic. The bad is that the poses start looking too similar to each other especially the faces and legs and the resin is a lot harder to clean up and paint than Knuckleduster or Dead Man's Hand. I will note that they still have the best collection of female figures, so if you are looking for that then don't pass them up.
As for Dead Man's Hand, I think they are kind of a middle ground. I've heard some people say they didn't like the quality of the sculpts, but I've painted almost every one from my lead pile which means that apparently I like their sculpts. They are easy to paint and have a wide variety of characters. The biggest downside is that they are pricey and they don't sell many individual figures. Previously, I think I would have said that they are better than Knuckleduster, but I think Forrest has eclipsed them now. While Knuckleduster has older movie characters, Dead Man's Hand has a lot newer ones (think Hell on Wheels, Coen Bros remake of True Grit, Django, even Westworld and more).
Of course, if you have the time and the money, the solution is to get a bit of everything. Then you get the variety that represents the diversity of humanity.
