Unity is huge in mobile gaming. UE4 barely made a dent on mobile until the recent mobile release of Fortnite that is, which requires a fairly high-end device
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.