The Spiller & Burr is a very nice piece, one of the few brass frame replicas I'd consider owning.
It has a top strap solid frame similar to the Whitney revolver, which predated the Remington.
The available powder space in the .36 chambers is a bit less than in the Colt Navy, so even a max compressed charge would not over stress the brass frame.
Power level is about the same as a .32 S&W Long.
These have had a good rep for accuracy and reliability, though I haven't seen any reviews on them since the 70's.
The Colt Root model is an interesting piece, is uses a side hammer like the Colt revolver rifles did.