Having used the µZynth for a few days I can report that such a small screen is uncomfortable to use.
- The font needs to be so small to fit much on the screen that even with my reading glasses and a magnifying glass I struggle to see some elements of the display.
- Fitting less on screen leads to more scrolling which make for a less fun experience.
- Status area is almost useless. The minimal approach leads to tiny elements that are indistinguishable from each other.
- Positioning of rotary encoders to allow comfortable control means they are not aligned with the screen and extend the vertical space required, reducing the advantage of the small screen. Having smaller (than 20mm) knobs may reduce this impact but may also reduce the ease of adjustment. (Small knobs on rotary encoders do not work very well.)
- Parameter control is obscured by multi-line labels.
- Top bar has insufficient vertical space to display scrolling scrolling and status info
So my assessment is that a 1.8" display is just about usable for a standard Zynth but is sub-optimal. Anything smaller would likely be useless (or extremely limiting). Such a small display may be advantageous for smaller form factor devices but these are likely to be used for performance where a larger display of the current patch is preferable.
My recommendation would be to consider a different UI for such small displays but this can present issues with code maintenance, documentation and user experience. It may be plausible to design a reduced size widget set to resolve some of the overlap issues. I think there should be a performance mode which displays the current patch in larger font which would address some of these issues if the small form factor device is primarily for performance only, i.e. not for sound design.
It may be appropriate to fix some of these issues with alternative display widgets and have a performance mode which is optimised for the display resolution in which case the could be a place for the µZynth but with current implementation I would recommend use of a larger display (than 1.8").