I do have experience with the module and contacting Synthrotek. Mix does have quirks which I assumed are from its passive nature and inexpensive 2hp design. Like, for example, turning unused channels all the way down when not used. Not turning the channels all the way up when using 2+ audio signals. It was like $25 so I can't complain. I still use it occasionally for audio but I have multiple active mixers for CV use. Their website says it's not suitable for additive voltages.
As for customer service, I contacted them for an incorrect order (they sent a DIY kit for Arcadian Rhythms instead of assembled) and they shipped out a new completed module by the end of the day. Got a free kit out of it.