This video will give you an idea of the complexity of these revolvers.

The main failures seem to be with springs. Good fitting aftermarket springs are pretty much non-existent. Replacement cylinder bolts are hard to find and require a lot of hand fitting.

 

Colt did sell 156,000 1877's so obviously a lot of people did like them. I own a number of them and they have a complicated action, but I think the stories of them being unreliable are exaggerated.
Jim

 

I love this gun, I own a few of them. I studied their design and repair them as a hobby.
Yes I think we can say "odd design" as Colt wanted to make a double action with keeping the single action design...
Poor parts are trigger spring with its tiny roller, the hand spring with its tiny pin, then the cylinder lock, the sear and their common double-leg spring.
What made them difficult to repair, I think is that when any part is worn out, the whole mechanism is affected, and adjustments are extremely delicate.
Anyway that is a very elegant revolver in my opinion !

 

It certainly is one of the strangest DA designs.

 

Is the larger frame 1878 any better than the 1877?

 

The 1878 is my favorite Colt double action after the 1877. It is stronger and more reliable than its predecessor, and its only delicate part is the hand, which must be replaced when the cylinder has too much play for a proper alignment of chambers with the barrel.

 

The Model 1878 had a larger frame, and is therefore sometimes referred to as the "large frame" double-action revolver, while the Model 1877 is likewise referred to as the "small frame" double-action revolver. The Model 1878 was a better design and was not as difficult to repair.
Jim