From what I'm thinking this has to do with their low latency routing model being different for instruments and busses and audio channels.
The thing that I bought into this platform for was their model/idea to separate the low latency settings from the buffer size. I even bought their hardware to get the hardware low latency for the extra couple ms. That was the dream for me until it came time for tracking of multi artist overdubs and cue mixing. I use an external personal mixing system (P16) so I don't need a mix per se in S1 but I just need a low latency pre-fader send of an un-muteable source that I can send to an output. So close but yet .....