But the FIA can't guarantee that anything is completely safe in Formula One so why should DRS be different? As I said before, the loss of some rear downforce from a DRS failure is much less dangerous than other parts failing on the car whose safety can't be guaranteed either.
It's desinged to fail closed, wasn't the reason Schumacher's DRS was stuck due to a hydraulics issue causing a tube to stick out, perhaps due to the parts Mercedes attach to their DRS rather than the DRS system itself. When he first came into the pits on lap 42 for his scheduled pitstop he had DRS activated passing Kobayashi and was able to brake from 317kph down to 100kph with it open without any incident or even knowing his DRS was still open at that point. Going out again with it still open was pretty dangerous though but that was their choice. All DRS zones are followed by a run-off area or gravel trap, so if it fails to close under braking the cars still have room to stop.
Unexpected DRS activation say mid corner in a race is perhaps a concern, but presumably the rest of the car eg. steering, brakes, throttle would still be working so a driver would hopefully be able to recover. Out of the numerous times DRS has been used we have so far only seen it fail to close once and I can only remember 1 occasion where it activated unexpectedly (Alonso in China last year), so I'd say it's pretty safe.