Just what I have heard. Having collies, and we all know about thier sensitive skin, I stay away from things I'm warned about. Heres some different bark collars.
Citronella Collar: This collar contains a reservoir of citronella solution that sprays into your dog’s face every time he barks. A citronella collar is considered humane and a recent study reported an 88% rate of success with the use of this collar. One possible drawback is that the collar contains a microphone, so the aversive is delivered in response to the sound of the bark. Therefore, other noises may set off the collar, causing your dog to be sprayed even if he hasn’t barked. Also, some dogs can tell when the citronella reservoir is empty and will resume barking.

Aversive Sound Collar: This collar emits a high-frequency sound when your dog barks. Some are activated by the noise of the bark, while others are hand-held and activated by a handler. The rate of success for this type of collar is reportedly rather low.

Electric Shock Collar: We don’t recommend an electric shock collar to control your dog’s barking. The electric shock is painful to your dog and many dogs will choose to endure the pain and continue barking. The success rate of this type of collar is less than 50%.

The main drawback of any bark collar is that it doesn’t address the underlying cause of the barking. You may be able to eliminate the barking, but symptom substitution may occur and your dog may begin digging, escaping, or become destructive or even aggressive. The use of a bark collar must be in conjunction with behavior modification based on the reason for the barking, as outlined above. You should never use a bark collar on your dog if his barking is due to separation anxiety, fears or phobias, because punishment always makes fear and anxiety behaviors worse.