If you wanted to run you own drivers, or had code written specifically against protocols exported by Tiano components I believe the answer is - yes.
RaspberryPiPkg is the $35 64-bit Arm UEFI devkit.
Of couse the U-boot implementation of UEFI spec is getting better and better day by day, and that’s obviously awesome. I am sure someone will add ACPI support to it any day now. Having choice is good, and I wouldn’t want to live in a world with a single implementation. That said, Tiano (like it or not) /is/ the reference implementation, and you’re much less likely to run into surprises (today). Notice how I didn’t say Tiano is /better/, because in a number of ways (code quality, complexity, fragmentation) it’s pretty awful.
One thing the U-boot implementation excels at is embedded Arm board support - U-boot is the defacto firmware for most embedded hardware, and now all of that hardware is EFI-capable. Porting Tiano is effort you don’t want to spend.
That said, there are hybrid approaches - EFIDroid (efidroid.org) runs Tiano on top of LK, using LK’s platform support for drivers.
I'd be interested to know what the memory footprint is like. I don't have an easy way to measure it.
Note that these numbers will also include Arm Trusted Firmware and firmware tables.
EDIT: I think that it was something along the lines of some implementations (including TianoCore) required the ExitBootServices call to be performed more than once to exit the UEFI-controlled environment correctly? Even after doing that I had some problems with it.
The specification doesn't say that an implementation must not change the memory map inside ExitBootServices, and in practice many runtime drivers clean up or even perform RT Data allocations in that path. This results in the "MapKey is incorrect" error returned to the caller. (I've seen some that modify ACPI and SMBIOS on the fly inside the ExitBootServices path, awful, awful, awful).
So yes, the program calling ExitBootServices must be written to retry the call even if it really did avoid sandwiching any calls to UEFI between the last GetMemoryMap and ExitBootServices.
The other big Achille's heel of UEFI in general is the SetVirtualAddressMap() RT call and RT Services support in general, but Tiano's driver model makes it particularly easy to shoot yourself in the face and end up with an implementation that forces the OS to call RT services with identity mappings or even with mappings for supposedly non-RT regions. Ugh. MMIO is a favorite one to "forget" to add to the memory map as an RT entry.
UEFI uses the PE format for binaries, but doesn't actually mandate how PE should be used or what the allowed relocations are. You could build a PE binary that amounted to, say, tons of executable segments, each of which will be a separate memory map entry. Now since UEFI doesn't mandate exactly how an OS will assign virtual addresses (for SetVirtualAddressMap), nor exposes relative grouping of memory descriptors (forcing the OS to relocate these keeping relative distances) I can imagine a situation where RT drivers could break if the OS decided to aggressively compact the address map or instead introduce random large VA gaps between RT memory map entries.
As hinted at in andreiw's reply, this is entirely spec-compliant (if really annoying). Quoting from the spec:
"Firmware implementation may choose to do a partial shutdown of the boot services during the first call to ExitBootServices(). A UEFI OS loader should not make calls to any boot service function other than GetMemoryMap() after the first call to ExitBootServices()."
The biggest possible barrier to that today, which is still unknown, is the quality of the USB host controller. NICs use bulk, but there's no guarantee that the host controller driver is good enough. Time will tell, I suppose...