As far as I can remember one of the biggest issue has always been her crit hitbox. Zarya’s beam is pretty accurate(given that you have good tracking) and is consistent(has no spread and can go for quite a while without loading) so a Zarya bubbling and running up to a D.Va is just a free de-mech plus kill if you can secure it.
Although that’s my experience/observation. Her DM being mediocre and her lack luster boost(I forget all of her changes but pretty sure her current boost is trivial compared to previous versions) are just cherries on top.