I work as a PC tech and i have clean installed Windows 8.1 x64 /Windows 10 x64 to PC/laptops from all major manufacturer's and custom built systems. I can safely say that all UEFI firmware will support and boot from MBR formatted USB flash drives (FAT32).
That would be of course "a large number of" major manufacturer's and custom built systems.
Black swans are rare, but they do exist.
However there are some buggy firmware which will only boot to UEFI mode if there is a valid UEFI bootloader is present on the USB stick. One particular example is newer Acer laptops. They will only boot to UEFI mode if "efi" folder is present on the root of USB stick (with EFI bootloader inside it) - even though boot mode is set to legacy only in BIOS setup. Only way to install either Windows 8.1/Windows 10 in legacy mode on that system is to delete or rename "efi" folder on the root of USB stick.
You mean "will only boot in non-UEFI mode if a valid UEFI bootloader is not present", I believe or - in other words - even if the boot mode is set to "legacy" in BIOS, if there is an EFI bootloader on the stick, than UEFI mode will prevail?
I mean, if there is no EFI bootloader the thingy won't boot to UEFI (of course).