I'm using ShipClassOverride on both base classes and classes that inherit from bases.
It appears that if you do not include the graphics in the base class override, the ships that inherit from that sometimes just...don't have graphics. Or in Kate Morgental's case, bafflingly, swap to a low facings version.
I have also had ships fail to inherit the ability to move, a ship completely refuse to override unless I used the old ShipClass method, and Kate Morgental not inherit her graphics from an overridden Eir and then revert to the old twenty-facing graphics for an unknown reason.
In short, we have quite a lot of problems with ShipClassOverride when you don't include everything, which partially defeats the purpose of ShipClassOverride.