Well, what would you rather barrel down a bumpy road in at 50mph, a seat with a 5 point harness, or no seatbelt and just a few pins to stick into your fire-proof pants? Riding South Mountain at speed did cause my feet to bounce off, not all at once, but significant hits and features would kind of get my feet closer and closer to the edge, causing me to have to reposition them, but the terrain is so unrelenting it's hard to lift a foot for anything without losing control. People do it though, so I know it's possible, it's just not for me (flats on technical terrain). For me, flats are for winter riding and high skinnies, and that's it.
But back to the topic,
For downhill, these are the types of pedals you want:
These have a plastic cage, not the most durable, but cheap (the mechanism is durable, just the cage is not). PD M434.
These are much stronger, kind of the "gold standard" in my years. A problem with this and the above is the end-cap unit strips in a few seasons/years of heavy abuse, which can be easily rebuilt with a new end-cap assembly (cheap). This was happening bashing rocks mind you and people behind me said they could see sparks flying off the pedals. Even when they "start to go" it doesn't render the pedal useless, the mechanism still works fine, just needs the end-cap. PD M545.
These are the newer version, with an acrylic/resin cage. These have a newer mechanism that clears mud easier. I never had the end-cap issue, as I think the metal cage on the above 545s transfers the forces just a bit more than these. In the rockiest craziest conditions the cage may not hold up, but they can be replaced and they are pretty strong. PD M647.
Even though these have a platform, I highly advise against these. They are much harder to get back into and to balance on if you miss the clip-in for downhill. The above pedals have a 30-degree spring-loaded cant to the mechanism that makes clipping back in easy, just put your foot down and it finds the mech as your foot slides on the pedal. In other words, the mechanism rotates WITHIN the cage. These don't work the same way and the platform is much smaller. You might see some of the advanced racer types with them, and you'll even see straight XC clipless pedals on their DH bikes too, but pass on these "clipless+platform" type pedals, known as "trail" pedals. Not good for a newer-to-DH type rider.