I consider MuTools MU.X to be a very powerful extension to Studio One when it comes to fancy instrument/effects chains in the direction of Bitwig, Ableton/M4L, or Reaper.
Something which severely bugs me though is that Studio One does not "see" the secondary audio input of MU.X as a sidechain target. Hence, you can't do sidechaining, envelope following, morphing, or anything else that relies on a second audio input within MU.X.
I personally have this issue with Studio One 4.1.3 and MU.X 7.7.4 on Windows 10, but this old (2013) KVR article https://www.kvraudio.com/forum/viewtopic.php?t=374530
seems to indicate that it's not specific to my configuration.
It should be mentioned that all other VST hosts I've tried show MU.X correctly as having two audio inputs. As a workaround, I can actually wrap MU.X in DDMF Metaplugin, offering two inputs towards the host, and passing them through to MU.X's inputs. On the other hand, my software engineering instincts tell me that the issue is probably easy to fix if you know Studio One's code for parsing the plugin's input/output properties ...