DX12 is very experimental with UE4 and the way it improves things doesn’t necessarily improve a game. The benefit is mostly with draw calls, which not all games have an issue with. In any case, none of those GPU’s are very good, they’re not designed for the purpose of games and game development, they are designed to get enough functionality for the average person and cut some performance so that it uses less power and less space.
What’s also a problem is the high screen resolution, most game stuff targets 1080p and stuff like the Surface 4 are higher than that, so if you run it at native resolution then you’re trying to run it at a resolution that’s higher than what people typically use. It’s a similar issue with phones–you’ve got phones with 1080p and 2k screens, and while a phone these days has much more power than like an Xbox 360 or a PS3 we don’t see games of that quality and part of the reason is that those console games ran at 720p resolution max and phones run much higher.