Basically, the safety needs to line up near or right on the LOS to make it home - he's not running in from 15 yards deep fast enough to make the play - and he either needs to be unaccounted for by the blocking scheme or some blocker needs to screw up their pickup. It's basically a play that needs to work with a free run or it's not going to work at all, so it's best used sparingly.
What teams like the Ravens and Patriots are starting to do is run the zero blitz - no deep safety, send 6 blitzers, trust everyone else in man coverage. Safety blitzes make up a huge part of this, as they are lined up in the box or over the slot, and it's ambiguous if they are going to cover or come on the blitz. The offense has a pretty good idea that 6 guys are going to come, but they don't know from where, and even if they do there's probably going to be a free runner (if a guy stays in to block, the defender covering him is supposed to realize it and come on the blitz). Safeties' speed and credible threat as either a blitzer or a coverage defender goes a long way to making the play work.
The play is designed to make a QB make a read FAST before the free runner gets there, and a lot of young or low-end QBs aren't ready to do that, so they take a sack or make a stupid throw. Then, the counter-punch to that once the offense thinks they can just instantly throw hot is to go to a max-cover look. You rush 3 and just flood the short part of the field with underneath zone coverage, and all of a sudden that hot route has 2 or 3 defenders crawling all over him. It puts offenses in a VERY tough bind, and in a league where free releases and offensive pass interference allows QBs to go from snap to throw in literally about 2.5 seconds, zero blitzes are one of the only ways to generate fast enough pressure to throw the play off.
Tl;dr: Belichick is a genius and shows the rest of the league how to run safety blitzes. He's still leading, everyone else is just following along.