On higher end pianos, our first instance is to think around needling, but being a delicate process, it is hard to justify the work on upright pianos that sound harsh.
I cam across the softener years ago, I assumed it was a lazy mans voicing. First tests were carried out with great caution, as we’d never used the product before, however with some experience, we can identity use cases where we know it will improve the tone greatly.
Use cases mostly compromise on entry level uprights, where needling might not be carried out due to high demands on time, and the softener can smooth the tone of the piano out for its next players enjoyment.