I see two aspects to this question.
- Braking system effectiveness when functioning.
- Reliability of braking system.
For 2. there is no comparison. Since cables brakes can be applied to both wheels, and having two brakes is always more reliable than one, cable brakes are a key component of a reliable braking system. And in fact, some bikes will have a combination of these two brakes, having a cable brake on the front and coaster brake on the rear. Now, you might think, gee, a cable can break, but a chain never will, WRONG. While I'm sure it is less common on chunkier chains found on fixed gear bikes, chains can and do break, but it is rare for two cables to break at the same moment.
For 1. there are several considerations. First and foremost again, because cable brakes can be applied to the front wheel, cable brakes can apply far more braking force to the bicycle than a rear wheel brake of any kind. When you apply the brakes, weight shifts to the front, off the rear wheel, meaning you can skid the rear wheel very easily. The front wheel, not so much. In dry conditions you can get 100% of your braking force from the front wheel. In wet conditions, you will benefit from having two brakes and get more total braking force by having both wheels braked.
As far as I understand, the main benefit of fixed gear bikes or coaster brakes is one of simplicity, they require less equipment on your bike and less maintenance. A secondary advantage is that the brakes themselves are not as affected by wet conditions. While rim brakes have significantly improved, they still function less effectively in wet weather. But rim brakes are not the only type of cable actuated hand brakes, you can in fact, use a hub brake (the same as a coaster brake) or a disk brake with a cable (or hydralics), and this in my opinion, for a city bike, is the preferred form of braking.