In mastectomy i use it mainly when performing skin or nipple-sparing mastectomy in order to diminished the heat delivered to the flaps and nipple... but to me the main use is in axillary clearance, as the time is less than with the traditional approach and suture less... There are reports that the rate of lymphedema is the same compared to a bovie or scissors axillary dissection. I haven't measure that in our practice but i have the impression that is correct...
I don't use harmonic shears much due to cost issues.
I normally infiltrate with a solution of saline/ levobupivacaine /adrenaline into a mastectomy plane and scissors dissect or diathermy for skin sparing mastectomy. Solution opens up plane and lessens risk of burns / bleeding.
Clearances - I use monopoly/ bipolar/scissors only.