What is the consequence for visibility when observing an extremely large, faint target with a moderate magnitude?

Its actual visibility plummets because the light is spread thinly across degrees of the sky.

A moderate visual magnitude rating for a celestial object can be misleading when dealing with extended targets like large nebulae or galaxies. Magnitude quantifies how bright an object appears as a point source or how bright its integrated light is across the entire field it occupies. If an object is extremely large and spans many degrees across the sky, the total light, even if moderate, is diffused over an enormous angular area. This diffusion results in a very low surface brightness. Consequently, even though the total light gathering might seem adequate based on magnitude alone, the resulting intensity per unit area is often below the threshold required for the human eye to register it against the terrestrial sky glow, causing the object’s practical visibility to decrease drastically.