I have this type of opening system on two different guns. One a Boss hammer gun like you mention and the second on a Scott hammer gun. Both guns were made in 1870's and have been well used. I have shot them only very limited amount as 135+ year old guns do not instill much confidence in their strenght and I have many others to play with. The thumb break as I call it works well and no problems. But if it fails I am sure some 137 year old design can be fixed by a good smith.
There was a Boss hammer gun with this system a while back that had been converted into a crossover shooter. Wicked looking gun that I should have bought just for laughs. It was in very nice shape and had seen little use after conversion. I suspect that a cross over gun like that gets very little use if the owner does not like it. Not like you can lend it to your brother or son. Need for crossover tends to be a one person only handicap.
As to the merits of the design it is a dead end of gun design evolution. Like the side lever it had a short life and went the way of many "odd improvements". Unless you intend to keep the gun long term or can get it for a decent price you need to consider how much you want the gun. Might be a little hard to sell down the road. Then again if you find a odd fellow like me maybe not.
