Many vintage Serotta frames use a proprietary windowed dropout with a connecting bearing insert at the seat stay ends. Like all moving parts on bicycles, these bearings need to be serviced regularly. In this particular case the bearings are an integral part of the dropout design. Elegant, light, and vertically compliant (!) this dropout design addresses performance more than durability and doesn't seem to do well if neglected.
Ti Cycles has developed a tool specifically for the service of Serotta dropout bearings, and there is no evidence to suggest that this dropout configuration is not reliable for recreational road riding if routinely looked after and the bearings are serviced every 3000 miles of normal use.
However, empirical evidence shows that this dropout is prone to developing cracks through one side of the dropout window as a result of impacts, the high stresses associated with racing, and/or lack of maintenance. While correlation does not explicitly equal causation, a very high percentage (pretty much all of them) of our sample set of bikes with "the Serotta crack" also have rear bearings long overdue for replacement.
"The Serotta crack" failure occurs more frequently on the upper piece of the drive side window but has also been found on the lower piece and/or on the non-drive side dropout.
Although the dropout design in question would greatly benefit from a little bit more material in the interest of long term durability, it would be unfair to consider them altogether inadequate. In most cases these bikes have been ridden for thousands of miles on less than pristine tarmac with the original, unserviced bearings in the seat stay connection to the dropout. Play in the rear dropouts via dead or worn bearings is likely to lead to an increased and irregular maximum point loading of the dropout window when under high stress.
For the past 30 years Ti Cycles has routinely repaired cracked titanium frames, including cracks through the windows on Serotta's proprietary dropouts. To date we have had an extremely high success rate with our repairs, including this one.
Because the frames are already well used and broken when we get them, we can't make any guarantees, especially for heavier riders or more aggressive use, but for general road riding we would expect the repair to hold for many years to come.