Is immunotherapy in asthma effective?

Yes. It is if this is the cause of your asthma. I'm a pulmonologist who regularly refers to allergists for this reason and have had the therapy myself. It works for many sufferers to reduce their symptoms (and sometimes eliminate them).
Controversial . In general if allergies are a trigger of your asthma and if you have skin tested proven allergic rhinitis then getting allergies under control would help to improve asthma control.
Can Be. If your asthma is triggered by allergic factors, immunotherapy may be very helpful. Consulting with an allergist may be helpful.