I spent a good share of my climbing with "just rope". It is more difficult to learn to do correctly and you really need to practice, practice and more. I personally think the belay it is quite safe, IF done correctly. You MUST anchor with the correct angle. Being tied around the waist is another thing.

A few years back I forgot my harness in the car after a hefty hike up to "Sugarloaf" rock climbing area. Rather than go back to the car, I belayed my partner (who outweighs me by a considerable amount) by "just rope". He fell, I caught him. Beth Rodden (a very famous climber) was next to us and amazed - being young, she had never seen this done.

The belay is not the problem. The bowline on the coil is a problem. A harness or even a more simplistic "diaper sling" is much better. If you fall with only a rope around your waist, you can easily die if you cannot get the weight off your ribs in a few minutes (that is what the prussik is for). You last a bit longer if you turn up-side down.

Learning the technique is only going to help you with regular practice. And you need to learn from someone who really knows what they are doing. Few climbers nowadays have been trained in this technique.