There are a number of reasons placing an outdoor digital display in direct and indirect sunlight is challenging. Here are a few design challenges:
Displays have to be very bright to be visible in the sun. Luminance should normally be 2500+ nits through the cover glass.
Displays should be backlit with high quality LEDs. Remember that LEDs experience degraded luminance capability over time.
Direct sunlight can cause the liquid crystal to boil and result in black blotching on the display, a phenomenon called Solar Clearing. The LCD needs to remain below this boiling point, which requires some mechanism for cooling the LCD.