I believe the ULF has no built-in cellular radio (i.e. BMW Assist functionality); it was designed explicitly to provide Bluetooth connectivity (& voice command recognition for navi). The TCU combines phone connectivity – whether a factory-installed handset or a Bluetooth-connected phone – & BMW Assist cellular hardware.
Additionally, the two units pair with a Bluetooth phone in different ways: the ULF uses a paring button (usually located under the armrest) to initiate the paring process, & the TCU starts up in pairing mode & remains discoverable for ~120 seconds.