From what I have seen on the EBT engines that we checked, 12, 14, 15, and 17, the boiler shells are quite good as far as thickness and consistency go. The backheads have a design issue that has manifested itself both during tourist service and before on one of the bigger engines (I forget which one, 16 or 18). This problem was corrected on #15.
The main issues are flue sheets and fireboxes, which in the overall scheme of things is normal wear and tear.
At this writing I can see no reason for replacing the boilers on 12 or 17.