Fibreglass has no problem at all with structural movement unless the building sways in the wind. A glass/resin mat is both flexible and copes with tension and pressure well. The greatest natural movement is the glassfibre expansion and contraction between full sunlight and freezing night and that doesn't cause problems on a properly made roof. Compared with the stresses on many glassfibre bodies and structures building movement under a roof is a complete non-event.
There are certainly many problems caused by poor quality construction done by roofers who have neither knowledge or experience of using it and who think you lay it like felt. I've seen fibreglass layed by roofers where they have placed one layer of thin fibre mat on the roof, poured buckets of resin on top and used a yard brush to spread it around a bit. That isn't going to survive anything. I've also seen roofers trying to lay it while it was raining. If you don't want to do it yourself ideally get a fibreglass worker from a local boatyard to do it for you!