Ed, I do a lot of work in partnership with a local plumbing outfit.
I'd have to say that it's only a question of 'when,' and not 'if', a sewer will fail. Even a perfect install, using premium materials and methods, will eventually fail. The biggest culprit seems to be the unever settling of the earth, which leads to reverse sloping of the pipe. Tree roots are another factor; when they don't actually enter the pipe, they move it about. Add to that the sundry materials that pass through sewers - especially when there is a clogging issue - and you have a finite life to the pipes.
This may surprise many, but a large percentage of the sewer pipes out there are made of cardboard. This material, common right after WW2, is working far past any estimated working life.
As for locating the pipe ... while I am sure the technology exists, the "call before you dig" folks do not look for sewers.
