meatwad makes a great point about the skinny rear tire braking. I personally am not usually comfortable riding a bike without a front brake. Anecdotally, i find coaster brake wheels with fat tires offer better braking than coaster brake wheels with skinny tires. But there are variables such as frame geometry and wheel diameter that are not accounted for.

It seems, however, a coaster brake would offer at least as good braking as any brakeless fixie with a good rider.