Even the piping codes shy away from evaluating systems after they are running.

Read B31.3 paragraph 300(c)(2): "This Code is not intended to apply to the operation, examination, inspection, testing, maintenance, or repair of piping that has been placed in service. The provisions of this Code may optionally be applied for those purposes, although other considerations may also be necessary."

John points out one issue - shakedown.

I've had users sing praises to CAESAR II, saying the numbers from the PC matched the field exactly. And I have also heard others complain about how poor the program is at predicting the current (structural) state of their piping system. If all models were accurate, I believe the happy users were looking at new construction while dissatisfied users were looking at systems with some history that included shakedown or other local yielding and/or some unintentional cold spring.