It might help to keep in mind that the display has not been designed to watch porn on. Expecting the same visual quality as the screen on your iPhone really isn’t reasonable.
The display is there to provide you with information when you need it. It does that much better than the previous LCD panels Elektron used because the contrast and viewing angle are both better.
Unevenly lit areas are a known downside of OLED panels, and can be seen on most phones with OLED displays when a solid color is shown (the display on the iPhone X is supposed to be the first one good enough for Apple).
This problem also exists in the OLED panel in the OP-1. That’s why the OP-1 user interface mostly uses line drawings and avoids solid areas of the same color.
As with all things, the choice for this display was a design and engineering trade-off made by Elektron.
I think they made a good choice because I watch my porn on my phone, but if you disagree you should by all means return the unit.