I can attest to the effectiveness of the only one I have tried - Histaminum Hydrochloricum 6. As a life-long asthma sufferer, dependent for too long on Albuteral/Steroid inhalers/Nebulizer, I found this tiny pellet under the tongue worked well when asthma/allergies flared. It stops the runaway histamine action, gently. I keep some in the fridge for any emergency allergic reactions like insect bites, etc. However, I have found attention to adrenals with herbs and adaptogens to be the ultimate solution - have not needed an inhaler/nebulizer or homeopathic pellet in a long,long time.
My research tells me that homeopathy is quite specific to symptoms - can be confusing and impractical.