It's not about the language, C# is generally fine if you know how to work the GC (which admittedly many people don't). The kind of C++ that is used for game code in UE4 isn't exactly "zero-overhead" either and it also uses garbage collection. Plus, on mobile C# in Unity is translated to C++ anyway. My guess is that the difference is negligible in most cases.
What really makes the difference is that Unity has light-weight rendering pipelines available, running on older GPUs. UE4 just ditches those devices.
What really makes the difference is that Unity has light-weight rendering pipelines available, running on older GPUs. UE4 just ditches those devices.