I played at an outdoor festival this weekend on a stage with a canopy, using a sunshade over the Stadium, with the brightness of the scribble strips set to 100%, and still I could barely read the scribble strips. The problem is two-fold. The OLEDs aren't nearly bright enough, and the glass picks up reflections so you have to move around to find an angle where you can read the text. The scribble strips for the active buttons were faint, but they could be seen. The OLED pixels for the inactive buttons are dimmed so they were nearly impossible to read even when I bent over for a closer look. The LED rings around the buttons could not be seen. Had it not been for the canopy, I believe that the Stadium may have been rendered unusable.
I submitted a support ticket a while back about the poor visibility of the scribble strips. I doubt that they can turn up the brightness much without damaging the OLEDs, but there are some things that could be done to improve the visibility. My suggestion is that they could create a "performance" setting which eliminates dimming of the inactive buttons. Instead, all OLEDs would be at maximum brightness, and the active buttons would have the pixel colors inverted to designate the buttons which are active. Something similar to the attached concept image. They still wouldn't have the high visibility of an LCD display, but it would be an improvement.
I love my Stadium, but I have a number of outdoor gigs this summer and if I end up in direct sunlight situation, the Stadium may be unusable. I play in three different bands and I'm one of those guys that creates a lot of song-specific presets, so memorizing what each button does in order to use the unit blindly is not an option. I really don't want to do it, but I plan to take my Boss GT-1000 as a backup to all outdoor gigs just in case the Stadium is impossible to use.