WiiMode is not emulated; it instead runs in a hardware-enforced sandbox. This results in certain Wii U mode features being available through exploits, such as Espresso code execution without Broadway compatibility mode enabled. None of these exploits allow a full Wii U mode takeover.
Because the Espresso boot ROM only boots ancast images, Broadway titles will not boot in their ordinary form. Instead, they must boot through a special apploader in ancast format, which then loads the title itself.
If an update is available for the Wii U, the Wii Mode will be disabled until the update is done, while IOSU255 runs.