More modern stress work goes thru an amazing set of intertwined analysis to try to ensure the design life of an airframe is achieved. Perhaps understandably, as the margins are cut closer by better methods, better knowledge of input loads and better lifetime stress monitoring, the result is that they take out more design weight rather than try to extend the airframe life. At least that was the case on the fighter/attack planes I have some knowledge of. The problem with this is that it is hugely iterative and expensive to do, along with the needed validation testing. But, it does make for some good airframes--until there is an unexpected load case, or one that was poorly understood. If you had a list of these, it would make for some very interesting stories about each airframe. Here are a couple that I know of:
--F/A-18C/D extended knife edge flight causes instability of the vertical stabilizers. I believe in the US this maneuver was somewhat restricted. There used to be a cockpit rearview video of an Aussie Hornet probably doing an airshow pass and you can actually see the large deflections. Loads greater than anticipated...?
--The LEX on the "legacy" F/A-18s originally caused a ton of issues. One of the "warts" that was added was a very robust fence on the top of the wing to try to get the air to behave. Otherwise, the buffeting loads (which are miserable to try to predict) were larger than expected.
--Of course, the buffeting of the tail on the P-38 which was ameliorated by a better wing root fairing. Buffeting probably wasn't even considered in the design.
--Not sure about the Stratofort, but on smaller aircraft, bomb release is actually a high stress load case even when flying straight and level. The situation for fighters is made worse by the use of external pylons with ejector racks. Perhaps a gravity drop from the bomb bay is benign--or not. How many pilots have described the plane "leaping up" on release?
--Test pilot Tex Johnson kind of annoyed (shocked?) the engineers responsible for the wing design on the B707 with his barrel roll. One of my relatives worked in the wing engineering area at the time...and it was a really big deal.
One reason that the USAF is extending the -52 is probably as a cruise missile launch platform. This legacy is interesting in that, as one of the last efforts by the -52G was as a cruise missile platform during operation Desert Storm in 1991--which is now an almost unbelievable 30 years ago. They carried up to six ALCMs on the two inboard pylons. If interested, go read about the Secret Squirrel program.
One might guess that if the US follows thru on development of hypersonic cruise missiles after being goaded into it by Russia and China, that these will also launch off a -52, but this time an H-model
Dave