What's so wrong with u-boot? Most of the trouble people have with it is "thanks" to chipset vendors who forked their version somewhere 10-ish years ago and completely mutilated it instead of putting in the effort to get their stuff upstreamed.
The only major thing it can't boot to my knowledge is Windows, but it might be doable if you chain-load Grub.
uboot needs to be modified to run a given operating system if it wasn't already supported (or a shim pretending to be one of the supported operating systems needs to be used). UEFI provides a way to support all operating systems through a standardized interface. uboot implementing UEFI interface provides the best of both worlds.
The only major thing it can't boot to my knowledge is Windows, but it might be doable if you chain-load Grub.