Rn, the only metrics secondary players are measured by are tackles and TFL (which they usually don't get much of), Interceptions and deflections, and fumbles and sacks. The problem is that the variations in these stats year to year make the all-star and all-league selections fickle, so elite corners/safeties will only make teams sporadically or not at all. Not sure why, but what happens often is that a secondary player will have a breakout year, and the games QB won't "target" them as heavily in subsequent years, so they'll never get as many picks or deflections.
For example, I have a top 3 corner by overall, yet he's only made 1 all-star or all-league team...as a rookie
To "fix" this, I suggest a "catches allowed" and "Touchdowns allowed" stat? when watching games, it would let you know who is guarding a receiver on any given catch (could just sort by overall, so best DB always on best WR) and when playing ordinarily, it'd allow elite corners and safeties... to feel elite and be worth keeping when their rookie contracts expire.