The only usability issues I’ve had with Mutable modules:
Peaks: I insisted on using Expert Mode to get the most out of it. But the knobs can’t be “read” – is that top knob showing the attack time of the envelope on channel 1, or the frequency of the envelope on channel 2? The only way to tell is to turn the knob, changing the current setting. And all too often, I’d just grab a knob and not realize I had the wrong channel selected.
Also I used the Dead Man’s Catch firmware, which adds a lot of cool functionality to it but required a cheat sheet to show what was selected. I find I don’t really like cheat sheets.
Warps: The Parasite firmware is a bit too cheat-sheet heavy for my liking; the control meanings change too much. The stock firmware is easy to work with though! (And the stuff Parasite offered, while pretty neat, wasn’t far off from effects plugins I have anyway.)
Frames: To some extent, once you start automating it, it has the “unreadable knobs” issue, but it’s less insidious than Peaks. The real problem I had was never remembering how to change the interpolation/response curves, how to switch from CV to sequencer control, etc. But mostly because I didn’t use those frequently – I found myself using the module like a giant Shades, for attenuation, offset and mixing, under manual control rather than automated. I decided those needs would be better suited by other modules…)
I never had any trouble with the interface of Tides, Rings, Streams, Plaits, Marbles, or Stages. Or Shades
I’ve briefly confused myself with Stages a few times, but the answer was always that I plugged in a gate somewhere to the left of segments that used to be independent, so I’ve gotten used to shuffling things as necessary if I have to rethink my patch.