Greetings to the list.
Years ago I took a class on gas welding for the machine shop course I was attending. It was a fun course, one of the things the instructor showed us was if you got the sheet metal too hot while brass brazing the brass would actuly flow through the grain structhure of the sheet metal. This happens in bicycle frames, I have had four frames break over the years, all in the heat effected zone. the bottom line is the skill of the operator.
The '72 Windsor that broke at the bottom head lug was not the only one, I have heard of at least three other ones breaking at the same place. The torelli Masi that broke at the seat tube BB shell joint and the TSX Bianchi that broke at the same place were operator or machine brazed problems. As was the Bob Jackson that broke at the fork crown fork blade joint.
I have been silver brazing for over 35 years, mostly scientific and medical equipment, a lot of high pressure or high vacuum parts. Ideally a silver solder joint should be no more than .002 to .003 wide, that's the industry standard in Silicon Valley, unfortunately thats not always the case then it's up to the skill of the operator .
Recently Ed Litton gave me some lugs and scrap tubing to practice on before I started my frame project, the joints were far from ideal for silver solder but flowing the silver was not a problem, after all these years, what is important is the surgical cleanliness of the parts and the flux.
In my opinion, if a frame breaks its normally from someone coming to work not focused on their work.