Why are several meshes really dark compared to all the others?

This looks like an Indrect Lighting Volume Sample issue. Go to show> Visualize> Volume Lighting Samples. Lightmass builds these samples in order to light Movable objects when they have no direct lighting. So for certain spots of your area, it decided that it was dark, and then projects that onto the entire model. That’s my first guess, but we’ll need more information about your scene. Movable lights? Static? Built lighting?