How is UE on mid-range and low-end mobile devices? I have the impression that's in increasing important chunk of Unity's end user base.
Interesting.
What really makes the difference is that Unity has light-weight rendering pipelines available, running on older GPUs. UE4 just ditches those devices.
It's not about performance in 99% of frames, it's about not dropping frames when the GC kicks in and needs more time than your budget accounts for.