what comes to my mind is:
excessive heat getting to the roots and
algae. light and moisture is a fine environment for them.

heat shouldn't be a problem. if it happens, it should dissipate rather quickly out the top. Algae, on the other hand, could present a few problems. While it won't directly consume your roots it does compete for nutrients and could potentially coat your roots and essentially smoother them.
It really depends on how bad it gets.
My EnF system has some algae, but not enough that I'm worried about it.

Most open top systems I've seen have been painted to block light from getting to the roots.

Except for only a few "water plants" ALL the plants we regularly grow using hydroponics normally grow in soil. As such, the roots are in total darkness.

Must be a reason for that!