I got a 1920x1080 screen (which I assume is probably the most popular screen format), and with a GUI size of 100% several things are fairly hard to read.
A size of 125% is kinda ok to solve this, but it’s that big that, when in “Advanced Mode” the GUI doesn’t even fit the screen anymore…
In the end both options don’t really work out in practice.
So at least one more option at about 110% or 115% would be highly appreciated.
To me the white GUI elements are very distracting.
The labels across the synth and the faders on the mixer are way too bright in context (the brighter knobs are ok-ish, as they are more into grey than white).
Not only that it doesn’t fit the overall theme aesthetically.
What’s far worse is that, because of the high contrast, these white elements jump right into your face. In particular the mixer faders are so prominent that it makes me feel like I start to squint when I look at the synth for a while… 
There should be “Mute” and/or “Solo” buttons on the OSC-Mixer.
No way of listening to a single OSC other than pulling down all other faders. Quite annyoing when you got four OCSs going… 
Pull them down… check your sound… try to remember where you had the faders before… dial them in again… realize you want/have to check again… repeat the same procedure… lol… awful PITA…
The slider on the Waveterrain OSC disappears when using custom images…
I can’t find any documentation about specs regarding the images to be used, not in the manual nor in the videos. Was hoping using images in square format might work, but nope.
Also seems the factory images are kinda hardcoded (or hidden in quite sophisticated manner lol…), so no way to look up how images are supposed to be…
EDIT: just for clarification, I got no idea if this is some graphical/technical issue (potentially the loading of a custom image triggering misbehaviour), or if this is happening because the images aren’t compatible 
