@mac64 , wrecks present totally different characteristics, and deep wreck penetration is usually more challenging (deep=you need to use the reel and are far from any natural light).
Regarding mine-diving, although unstable mines exist, this is not necessarily the case; furthermore, unstable caves exist too. But these places are generally not for novices; therefore, they are not even considered in the course standards (no mentally stable instructor would ever bring the students into an unstable environment). To be clear, by unstable, I mean that if you touch something wrong, the environment change (for instance, a wall could collapse).
Regarding the differences between caves and mines, the critical aspect is that mine navigation is generally more straightforward than cave one. This is why, according to most standards, cave divers can go into mines, but not the other way around.
EDIT: there are obviously some other differences between mines and caves, but they are less critical than navigation.