Directed (in essence) means that the National Academies (NAS) decadal survey recommended that specific mission. (They specify the what but not too much about the how.) A directed mission is assigned to an implementing center (JPL , GSFC, etc.) And they build it to accomplish their best interpretation of the NAS DS guidance.
Sometimes this can be expensive, more expensive than the NAS expected (JWST). And sometimes the imperative to make it work to accomplish the “down from the NAS” guidance, with near 100% certainty, in a novel environment will result in over performance like you noted.
The noteworthy directed missions are mostly large flagship missions.
The other category is cost capped competed missions. (VERITAS is a recent one, so was New Horizons, but they exist in astrophysics and earth science as well as planetary; see for example https://en.m.wikipedia.org/wiki/New_Frontiers_program). “Tell us what you can build to accomplish some portion of the following high level science goals in $300M or less”
Over engineering is not really possible under those conditions! You will be competing against the absolute best concepts and teams in existence. So the budget is really squeezed.
These two mission types are complementary in the obvious way. The flagships push engineering capacity way forward, and the competed missions have a lot of innovative concepts within a cost cap.
There are also pure tech demonstrations (often piggybacking like the helicopter did) and certain other categories but the competed/directed split is primary.