I've been using my UCX 2 for some surround sound applications and it works well, both with Windows and with Nuendo and there's no issues getting a surround signal to play, and to route to the right channels. However, I can think of three features in Totalmix that would make it a little easier/more useful if they are feasible to add:

1) Bass management for channels and a sub. There's no good way that I can figure out with the EQ as it is set up to do bass management, where you highpass the frequencies going to a main channel and send the lowpass to the LFE channel. It would be nice since, much as I would love to own full range speakers, I've neither the budget nor space for them in my tiny home studio. While you can, and I do, bass management with VSTs, it would be more convenient to just have the interface itself handle it since it is something you want "always on" and with every application. I would suspect the DSP has enough power to implement the necessary EQs, but I don't know for sure. If so, it would be a nice feature.

2) The ability to set up a unified "surround" output with level controls for the channels, rather than managing things as multiple pairs. Basically, allow the main output to be set to 6, 8 (or maybe more with the new immersive formats) channels and have a single master fader control it all. Then have level tweaks for each channel in that interface. It is not necessary, of course, you can do all that now and just group the output faders to all work in tandem with an ARC unit, but it would be easier and more clear if it were a single interface for all the channels, as you see in DAWs.

3) Delay settings on channels to time align them for surround mixes. Basically change and repurpose the echo effect to instead be able to add a few ms delay on channels, as needed to keep them time aligned. Again, something that can be done with a VST, but would be nice to just have always set for all applications.

None of these are critical features, but all would make the surround experience nicer.