Shayla, We did tons of these in our photo lab. As a test we put a couple outside on our sidewalk sign. They did fine in the winter but when it got hot the fading was noticeable.
We asked the manufacturers about it an it seems the dyes are sensitive to heat far more than light. It makes sense since it is a heat process that puts the images on the metal.
Called dye sublimation. The images we had in the store didn't seem to be affected by not having glass over them. Unless you use museum glass, I think some of beauty is lost behind glass. We have some at home now I can't really see any fading but I don't have anything to compare directly. Same problem as with any artwork and no glass, indoor pollutants such as smoke, grease and dust will affect the quality. I wouldn't recommend cleaning with anything but cool water.