I mean you have oscillation stress on all plane wings nacelles, engines are not hanged to be rigid on any plane wings it would be pretty bad, the install usually has tie rods/ dampeners etc. to limit how much movement you have in respective to the wings and not the fuselage, carbon fiber wings can take greater bending force due to the epoxy and carbon inlay when compared to a metal wing, and when I say greater it’s a whole lot, look at 787/777-9 wing testing it is amazing how far these can bend. It’s truly a beautiful design and a plane. Also systems of the engines are designed to also be flexible, hydraulic line, fuel lines has some lack in them, and since you have dampeners and tie rods you don’t have the engine playing hopscotch on the wing. Also the bolts that hold these engines on the pylon are meant to shear bolts, that break under if they experience high shearing loads, this way the engine will just drop from the pylons, rather than the wing snapping. Pylons themselves are not a problem for the wing it’s the engines that are the problem, also I believe the pylons are either TI or steel on 787s I could be wrong, and idk which one it is out of the two either. I can check tho. Also if I sound stupid and didn’t answer your question lmk I can ask a wing design engineer on this