As an expert in laser surgery, I am frequently consulted to evaluate poor scars. First, there is a difference between hypertrophic and keloid scars. Hypertrophic scars are usually red and raised but are confined to the incision. Keloid scars generally occur in darker skin patients and extend beyond the incision or zone of injury.
In this case, your surgeon attempted to limit the visibility of the scar by placing it in the armpit or axilla. Predicting healing of scars is very difficult so there may not have been any way your doctor could have predicted the poor scar. However, it is fortunate that the scar is in the axilla and not across a more visible part of the arm.
Other factors which can contribute to poor scar formation are genetic tendency, skin type, previous history of poor scars, and tension on the scar. In the future, please mention this poor scar to other surgeons who may be involved in your care. The tightness of the brachioplasty or armlift procedure probably contributed to the poor scar.
My advice would be to continue the steroid injections every 6 to 8 weeks. I usually add a series of Pulsed Dye Laser treatments in addition to the steroid injection. The laser will target the vascularity in the scar causing it to appear less red and perhaps cause you less pain or discomfort. Usually, it takes several steroid injections and laser treatments before you see an improvement. You may also try a silicone sheeting product such as Mepiform which can be worn under clothing. Finally, you should massage the scar several times a day which will break up the scar, prevent worsening of the scar, and eventually soften the scar.
Please don't get too discouraged. It sounds like you have a very good doctor who can help you through this healing process which will take many, many months. Scars do improve with time and these suggestions are a way to optimize your situation.