The saucer and hull sections on the Polaris class are at different heights compared to the other classes. When mixing and matching them, the difference is very noticeable:
With a Luna saucer on a Polaris hull, a good section of the hull is buried, including a weapons port, and upper parts of the hull obviously not meant to be seen are clearly visible;
Worse than this is using a Polaris saucer with a Luna hull- aside from the odd protrusion of the hull on the underside of the saucer, when the ship is viewed from directly behind, a gap is visible where the saucer meets the hull- you can literally see through the saucer.
I have a screen shot, but can't post attachments, will do so when possible.