While the two formats are very similar they are NOT identical or compatible. The MCU communication protocol includes pretty much everything that the HUI had plus some further advances, and most DAW software supports MCU, whereas software that never had HUI support in the first place is probably never going to have it at this point since the hardware has been discontinued for many years. The HUI also had a couple of mic preamps as well, so in the end it was a similar combinations of features to what you'd find on today's SSL Nucleus.
The HUI was larger and bulkier than the MCU and was also a monitor controller, so it had audio ins and outs for connection between your DAW and your speakers.
Later versions called Mackie Control Universal (aka MCU) appeared once other software began supporting the Mackie Control protocol, which was originally developed by Mackie and Emagic specifically for Logic.
The Mackie Control was at first a dedicated controller for Logic, and the front panel labels reflected this. HUI was the first controller from Mackie and is very old.