I'm uncomfortable with the bicycling aids, and prefer to hold the leash in my hand as I ride the bike, so I can keep the dog in position. I also want the dog to be right where I can see exactly what he's doing, so I can anticipate a problem. I can't turn my head much, so the aids put the dog back where I can't see and anticipate. I think the bike is important - wobbly wheels or badly adjusted steering or brakes....this doesn't go well with exercising dogs. The brakes especially should be very reliable and stop the bike without tipping the bike - especially the front brake. One hand brake and a coaster brake is the best solution for having one hand occupied.
I'm extremely strict about the dog not crossing in front of the bicycle - just a touch to the front wheel can cause the bicycle rider to do a serious face plant.
if the dog really isn't getting it and is endangering the rider, well, it isn't always possible to exercise the dog that way. Especially if the person would be getting more than scraped or bruised...not a good idea for older folks, though having the dog pull a wheel chair is usually a hoot, as long as the dog can be controlled with the collar and leash.