I love the "organic" quality of in-camera miniatures, such as LOST IN SPACE or SPACE: 1999, but mixing them with opticals might have looked discontinuous.
Actually, Space:1999 did mix the methods. When taking off, flying through atmospheres and landing on surfaces, the Eagles were models on wires on tabletop sets in front of backdrops. However, for the most part, Eagles in space were composite shots. I don't have an image to put up, but if you get an opportunity to pop in an episode, space shots of Eagles and other crafts usually are in a part of the screen with no stars. There could be a starless path or the bottom third, say, of the screen would have no stars and the ship would be in that area. This way no stars would show through the shadowed portions of the models.
In the first episode of the second year, The Metamorph, an Eagle breaks free of a magnetic force. we see it pivot and tilt away from the camera, but we can spot stars showing through the dark areas of the miniature.