Here's my understanding....

Strength - This answer depends on what you mean by strength. Traditional CrMo gets brittle if overheated. Hence the need to handbraze it at low temperatures. Tubesets like Mangalloy, Mangy X and Fuji's Valite (same as Mangy X?) don't get as brittle when heated, so a functional joint could be machine brazed by the robotics of the 80s. I've no data to back it up, but I suspect that the final joint integrity (tubes pulling out of lugs) and final brittleness of the tubes (tubes sheering) would be about the same. Other aspect of strength are the tube's ability to resist dents and to resist being bent. Again, I would expect the tube sets to be roughly equivalant.

Current Production - IMO, lugged frames (which I dearly, dearly love) offer no real value other than aesthetics. [1] And if you are buying a lugged steel frame on the basis of aesthetics, handbuilt frames offer more appeal to most folks. It's no surprise to me lugged bikes continue to flurish among the high end custom and quasi-custom builders. But in terms of factory machine built bikes, machine brazed lugged frames were displaced by internally lugged AL, welded AL and Carbon frames. In particular, the current welded AL frame offers less weight and more stiffness for less money. And, coming back around to the aesthetics thing... I think the majority of recreational riders want to buy bikes that look like racing bikes, which means they need to look like fancy AL or Carbon. People buy with their eyes and hearts, not their heads.

I would be interested in T-Mar's take.

[1] - Some will argue that lugged tubes offer the possibilty of repair, which is true. But, it is very costly and I personally don't see sending a machine brazed frame to a frame builder for repair when a perfectly functional machine made frame could be purchased new for a competitive price.