Even with enough practice and leg strength you'll never match the braking power of a proper rim/disc brake.
Sure it's easy to lock the rear wheel (at any speed) but skidding is far less effective at slowing you down than progressive non-skidding braking.
Simply put, I'd never go brakeless fixed. Even on the flat I go fast enough that I wouldn't be able to stop in a useful time/distance by leg-braking alone.
I was descending Broomfield Hill in Richmond Park the other day and a deer wandered out into the road (they usually sit close by ignoring all of the passing traffic). I usually hit 55kph going down that hill. Leg braking on a 10% descent on wet smooth tarmac is going to do close to nothing. Locking your back wheel will send you into (a) an oncoming car, (
the deer or (c) off the road. None of which you want to do at 20kph let alone 50kph.
Fit a front brake and use that to modulate your speed on fast descents, and for emergencies.
Sure it's easy to lock the rear wheel (at any speed) but skidding is far less effective at slowing you down than progressive non-skidding braking.
Simply put, I'd never go brakeless fixed. Even on the flat I go fast enough that I wouldn't be able to stop in a useful time/distance by leg-braking alone.
I was descending Broomfield Hill in Richmond Park the other day and a deer wandered out into the road (they usually sit close by ignoring all of the passing traffic). I usually hit 55kph going down that hill. Leg braking on a 10% descent on wet smooth tarmac is going to do close to nothing. Locking your back wheel will send you into (a) an oncoming car, (

Fit a front brake and use that to modulate your speed on fast descents, and for emergencies.