For what its worth, I think thats generally the issue with playing the safety though.
Ideally as a "safety" you should only be making a play because the CB either lost his man in coverage, or the front 7 didnt make the tackles to stop the runner, so if YOU screw up as the "safety" then really there is no one left to stop the play.
I think playing the safety position allows you to MAKE plays (INT's, sacks, FF) but it also allows you to GIVE UP big plays too. I think its the most "risk/reward" position to play.
Only situations I like playing the safety is;
1) against human players who ALWAYS roll out to buy time, and throw deep 75% of the time
2) against CPU when I have ELITE CB's and am playing a relatively weak WR tandem and can afford to gamble knowing my guys on the outside will lockdown the opponents WR's so I can try to take advantage of bad passes, or poor pass blocking.
Some people are really good at playing the safeties... I just find that its too easy to take a bad angle at times, and I absolutely despise giving up big plays. Playing MLB's seems to allow more room for error, and I just switch to the safeties on the deep ball or only AFTER I've gotten clogged up at the line on a toss play/option and the safety is the only guy left to make the tackle.
I always do have a special admiration for the guys who can play the safeties very well though.