There are few safeties that hold value from year to year. Really, there's probably only about 10 DBs a year that you can plug in and forget for the rest of the year, and invariably some of them will disappoint. In general, I try to identify short-term solutions that share these traits:
-Safeties that spend significant snaps in run support. Usually SS, of course, but not every SS. It depends on scheme.
-Safeties on teams with weak tackling LB corps, because otherwise LB will suck up tackle opportunities.
-Safeties on teams that like to blitz or preach aggressive philosophies. Sacks and forced fumbles can go a long way towards mitigating those DB clunker weeks, whereas some DBs settle into a very boring 3-0-0 routine with no chance of ever doing anything to spike their production.
I've had a lot of luck basing my drafts on those characteristics, and letting other people chase DBs that may have more points, but have been a one-year wonder. This year, for example, I went after George Wilson and Kam Chancellor in almost every league--and got them, while everyone else was taking more familiar names.
Regarding talent/opportunity, I've always felt talent is less necessary the further back you go in the defensive backfield. I think DLs require the most talent to be effective, LBs slightly less. DBs, however, often suck. They get tackles because they are relentlessly targeted or run at by offenses that perceive them as a weak link. You should familiarize yourself with the "rookie corner rule", and always be on the watch out for new starters that may be one-year godsends. Sometimes, even shorter periods of time. A.J. Jefferson was awesome for me the weeks he was returning kicks, but I didn't hesitate to drop him once Stephens-Howling came back and reclaimed his job. You should almost always find a mass of serviceable DBs on the waiver wire. The best suggestion is 1) don't trust interceptions, 2) figure out how many snaps they play/what role they play, and 3) be aware of who they've played already. Picking up a nickel back with a lot of tackles is great, but if they just played Green Bay and New Orleans, their stats may be severely skewed.