What to improve in Unreal Engine 4 from a Unity Developer's point of view?

Agreed, I .MP3 support would likely be useful for a lot of folks. I probably wouldn’t use it in a build.

A better idea would be to convert the .mp3 to .wav on import, so there’s no cross-platform compatibility issues or anything. There’s a lot of open-source converters out there too, so the code wouldn’t be difficult to find.