Knee health is a topic dear to my heart. I had a meniscus removed age 16 and since then my left knee has grown in boniness to be 2" bigger than its counterpart. It's got to the point now when the only time it's really comfortable is cycling! After a year of FG at 72" (with two centuries) I find no more aches and pains than with gears. I find that if you relax into downhills you get a massage effect on your knees which can be quite delightful. As for mashing: get off the seat and maintain momentum using all those core muscles. This has a wonderful effect on a beery waistline too! On that note my wife now rides SS and has the best shape she's ever had.....aged 45!
Safety and brakes: never a problem. Don't push back when stopping, take your time, relax and get that massage effect, the weight of your legs helps the braking. Sheldon Brown was absolutely right (as usual), in that FG riding makes you a better, smoother, safer cyclist.
Incidentally I'm doing a 24hr endurance ride at Thruxton Park motor racing circuit this year. Rules dictate that I can't use FG so I'm going to go to a 3-speed hub. Training will all be done on FG so I'll let you know the double centuries go! Happy Christmas from a very very wet England.