Although head collars have many advantages, they also have some disadvantages. To begin with, if you get carried away too much, the dog can become dependent on the tool, and not apply the training that he has received without it. Dogs can sometimes quickly identify the differences between their normal collar and head collar, and will adjust their behavior accordingly to the one they are wearing.
Also, there are some canines, particularly those that may not be used to wearing a collar on their head, who will dislike it so much that they will continually kick, scratch, or continue to pull on it excessively. If you are trying to use one with an unruly dog that is doing this, your best bet is to keep the dog moving and distracted until he gets used to wearing a head collar. You can also make the dog sit by pulling on it as another corrective movement.
There is also a minor issue with the head collar. Many people will react and treat it as a muzzle, when in fact it is not. This causes people to react to the dog as if it might bite them. This is not a huge setback, some find it worrisome.
In general, if you plan to wear a head collar for training, do the following. Read the instructions that come with the collar first. Have a game plan for dog training and be sure to use good solid training methods in combination with the collar. Just using this tool alone is not a life saver.