Only removable disks use bootx64.efi, though some (but not all) BIOSes will look for that on fixed disks too.  efibootmgr needs access to efivarsfs to configure an EFI boot entry for Linux on a fixed disk.  Relying exclusively on bootx64.efi on the EFI system partition will break on some devices, and will overwrite the Windows bootloader (and conversely, Windows doing so will render Linux unbootable).